--- a/docshell/base/nsDocShellTreeOwner.cpp
+++ b/docshell/base/nsDocShellTreeOwner.cpp
@@ -1213,17 +1213,17 @@ ChromeTooltipListener::MouseMove(Event* 
 
   if (mTooltipTimer) {
     mTooltipTimer->Cancel();
   }
 
   if (!mShowingTooltip && !mTooltipShownOnce) {
     nsIEventTarget* target = nullptr;
 
-    nsCOMPtr<EventTarget> eventTarget = aMouseEvent->GetTarget();
+    nsCOMPtr<EventTarget> eventTarget = aMouseEvent->GetComposedTarget();
     if (eventTarget) {
       mPossibleTooltipNode = do_QueryInterface(eventTarget);
       nsCOMPtr<nsIGlobalObject> global(eventTarget->GetOwnerGlobal());
       if (global) {
         target = global->EventTargetFor(TaskCategory::UI);
       }
     }
 
@@ -1312,16 +1312,22 @@ ChromeTooltipListener::HideTooltip()
 //   -- the x/y coordinates of the mouse      (mMouseClientY, mMouseClientX)
 //   -- the dom node the user hovered over    (mPossibleTooltipNode)
 void
 ChromeTooltipListener::sTooltipCallback(nsITimer* aTimer,
                                         void* aChromeTooltipListener)
 {
   auto self = static_cast<ChromeTooltipListener*>(aChromeTooltipListener);
   if (self && self->mPossibleTooltipNode) {
+    if (!self->mPossibleTooltipNode->IsInComposedDoc()) {
+      // release tooltip target if there is one, NO MATTER WHAT
+      self->mPossibleTooltipNode = nullptr;
+      return;
+    }
+
     // The actual coordinates we want to put the tooltip at are relative to the
     // toplevel docshell of our mWebBrowser.  We know what the screen
     // coordinates of the mouse event were, which means we just need the screen
     // coordinates of the docshell.  Unfortunately, there is no good way to
     // find those short of groveling for the presentation in that docshell and
     // finding the screen coords of its toplevel widget...
     nsCOMPtr<nsIDocShell> docShell =
       do_GetInterface(static_cast<nsIWebBrowser*>(self->mWebBrowser));

--- a/dom/webidl/Element.webidl
+++ b/dom/webidl/Element.webidl
@@ -261,16 +261,20 @@ partial interface Element {
   [BinaryName="shadowRootByMode", Func="nsDocument::IsShadowDOMEnabled"]
   readonly attribute ShadowRoot? shadowRoot;
 
   [ChromeOnly, Func="nsDocument::IsShadowDOMEnabled", BinaryName="shadowRoot"]
   readonly attribute ShadowRoot? openOrClosedShadowRoot;
 
   [BinaryName="assignedSlotByMode", Func="nsDocument::IsShadowDOMEnabled"]
   readonly attribute HTMLSlotElement? assignedSlot;
+
+  [ChromeOnly, BinaryName="assignedSlot", Func="nsDocument::IsShadowDOMEnabled"]
+  readonly attribute HTMLSlotElement? openOrClosedAssignedSlot;
+
   [CEReactions, Unscopable, SetterThrows, Func="nsDocument::IsShadowDOMEnabled"]
            attribute DOMString slot;
 };
 
 Element implements ChildNode;
 Element implements NonDocumentTypeChildNode;
 Element implements ParentNode;
 Element implements Animatable;

--- a/dom/webidl/Text.webidl
+++ b/dom/webidl/Text.webidl
@@ -16,11 +16,14 @@ interface Text : CharacterData {
   Text splitText(unsigned long offset);
   [Throws]
   readonly attribute DOMString wholeText;
 };
 
 partial interface Text {
   [BinaryName="assignedSlotByMode", Func="nsTextNode::IsShadowDOMEnabled"]
   readonly attribute HTMLSlotElement? assignedSlot;
+
+  [ChromeOnly, BinaryName="assignedSlot", Func="nsTextNode::IsShadowDOMEnabled"]
+  readonly attribute HTMLSlotElement? openOrClosedAssignedSlot;
 };
 
 Text implements GeometryUtils;

--- a/layout/xul/nsXULTooltipListener.cpp
+++ b/layout/xul/nsXULTooltipListener.cpp
@@ -97,17 +97,23 @@ nsXULTooltipListener::MouseOut(Event* aE
     return;
 #endif
 
 #ifdef MOZ_XUL
   // check to see if the mouse left the targetNode, and if so,
   // hide the tooltip
   if (currentTooltip) {
     // which node did the mouse leave?
-    nsCOMPtr<nsINode> targetNode = do_QueryInterface(aEvent->GetTarget());
+    EventTarget* eventTarget = aEvent->GetComposedTarget();
+    nsCOMPtr<nsIContent> content = do_QueryInterface(eventTarget);
+    if (content && !content->GetContainingShadow()) {
+      eventTarget = aEvent->GetTarget();
+    }
+
+    nsCOMPtr<nsINode> targetNode = do_QueryInterface(eventTarget);
 
     nsXULPopupManager* pm = nsXULPopupManager::GetInstance();
     if (pm) {
       nsCOMPtr<nsINode> tooltipNode =
         pm->GetLastTriggerTooltipNode(currentTooltip->GetComposedDoc());
       if (tooltipNode == targetNode) {
         // if the target node is the current tooltip target node, the mouse
         // left the node the tooltip appeared on, so close the tooltip.
@@ -168,17 +174,21 @@ nsXULTooltipListener::MouseMove(Event* a
   // so that the delay is from when the mouse stops moving, not when it enters
   // the node.
   KillTooltipTimer();
 
   // If the mouse moves while the tooltip is up, hide it. If nothing is
   // showing and the tooltip hasn't been displayed since the mouse entered
   // the node, then start the timer to show the tooltip.
   if (!currentTooltip && !mTooltipShownOnce) {
-    nsCOMPtr<EventTarget> eventTarget = aEvent->GetTarget();
+    nsCOMPtr<EventTarget> eventTarget = aEvent->GetComposedTarget();
+    nsCOMPtr<nsIContent> content = do_QueryInterface(eventTarget);
+    if (content && !content->GetContainingShadow()) {
+      eventTarget = aEvent->GetTarget();
+    }
 
     // don't show tooltips attached to elements outside of a menu popup
     // when hovering over an element inside it. The popupsinherittooltip
     // attribute may be used to disable this behaviour, which is useful for
     // large menu hierarchies such as bookmarks.
     if (!sourceContent->IsElement() ||
         !sourceContent->AsElement()->AttrValueIs(kNameSpaceID_None,
                                                  nsGkAtoms::popupsinherittooltip,

--- a/toolkit/components/tooltiptext/TooltipTextProvider.js
+++ b/toolkit/components/tooltiptext/TooltipTextProvider.js
@@ -4,21 +4,20 @@
 
 ChromeUtils.import("resource://gre/modules/XPCOMUtils.jsm");
 ChromeUtils.import("resource://gre/modules/Services.jsm");
 
 function TooltipTextProvider() {}
 
 TooltipTextProvider.prototype = {
   getNodeText(tipElement, textOut, directionOut) {
-    // Don't show the tooltip if the tooltip node is a document, browser, or disconnected.
+    // Don't show the tooltip if the tooltip node is a document or browser.
+    // Caller should ensure the node is in (composed) document.
     if (!tipElement || !tipElement.ownerDocument ||
-        tipElement.localName == "browser" ||
-        (tipElement.ownerDocument.compareDocumentPosition(tipElement) &
-         tipElement.ownerDocument.DOCUMENT_POSITION_DISCONNECTED)) {
+        tipElement.localName == "browser") {
       return false;
     }
 
     var defView = tipElement.ownerGlobal;
     // XXX Work around bug 350679:
     // "Tooltips can be fired in documents with no view".
     if (!defView)
       return false;
@@ -118,17 +117,28 @@ TooltipTextProvider.prototype = {
               break;
             }
           }
         }
 
         usedTipElement = tipElement;
       }
 
-      tipElement = tipElement.parentNode;
+      let parent = tipElement.parentNode;
+      if (defView.ShadowRoot &&
+          parent instanceof defView.ShadowRoot) {
+        tipElement = parent.host;
+      } else {
+        let slot = tipElement.openOrClosedAssignedSlot;
+        if (slot) {
+          tipElement = slot;
+        } else {
+          tipElement = parent;
+        }
+      }
     }
 
     return [titleText, XLinkTitleText, SVGTitleText, XULtooltiptextText].some(function(t) {
       if (t && /\S/.test(t)) {
         // Make CRLF and CR render one line break each.
         textOut.value = t.replace(/\r\n?/g, "\n");
 
         if (usedTipElement) {

--- a/toolkit/components/tooltiptext/tests/browser.ini
+++ b/toolkit/components/tooltiptext/tests/browser.ini
@@ -1,8 +1,9 @@
 [browser_bug329212.js]
 support-files = title_test.svg
 [browser_bug331772_xul_tooltiptext_in_html.js]
 support-files = xul_tooltiptext.xhtml
 [browser_bug561623.js]
 [browser_bug581947.js]
 [browser_input_file_tooltips.js]
 skip-if = os == 'win' && os_version == '10.0'  # Permafail on Win 10 (bug 1400368)
+[browser_shadow_dom_tooltip.js]

new file mode 100644
--- /dev/null
+++ b/toolkit/components/tooltiptext/tests/browser_shadow_dom_tooltip.js
@@ -0,0 +1,131 @@
+/* eslint-disable mozilla/no-arbitrary-setTimeout */
+
+add_task(async function setup() {
+  await SpecialPowers.pushPrefEnv(
+    {"set": [["ui.tooltipDelay", 0],
+             ["dom.webcomponents.shadowdom.enabled", true]]});
+});
+
+add_task(async function test_title_in_shadow_dom() {
+  let tab = await BrowserTestUtils.openNewForegroundTab(gBrowser);
+
+  info("Moving mouse out of the way.");
+  await EventUtils.synthesizeAndWaitNativeMouseMove(tab.linkedBrowser, 300, 300);
+
+  info("creating host");
+  await ContentTask.spawn(tab.linkedBrowser, {}, async function() {
+    let doc = content.document;
+    let host = doc.createElement("div");
+    doc.body.appendChild(host);
+    host.setAttribute("style", "position: absolute; top: 0; left: 0;");
+    var sr = host.attachShadow({ mode: "closed" });
+    sr.innerHTML = "<div title='shadow' style='width: 200px; height: 200px;'>shadow</div>";
+  });
+
+  let awaitTooltipOpen = new Promise(resolve => {
+    let tooltipId = Services.appinfo.browserTabsRemoteAutostart ?
+                      "remoteBrowserTooltip" :
+                      "aHTMLTooltip";
+    let tooltip = document.getElementById(tooltipId);
+    tooltip.addEventListener("popupshown", function(event) {
+      resolve(event.target);
+    }, {once: true});
+  });
+  info("Initial mouse move");
+  await EventUtils.synthesizeAndWaitNativeMouseMove(tab.linkedBrowser, 50, 5);
+  info("Waiting");
+  await new Promise(resolve => setTimeout(resolve, 400));
+  info("Second mouse move");
+  await EventUtils.synthesizeAndWaitNativeMouseMove(tab.linkedBrowser, 70, 5);
+  info("Waiting for tooltip to open");
+  let tooltip = await awaitTooltipOpen;
+
+  is(tooltip.getAttribute("label"), "shadow", "tooltip label should match expectation");
+
+  info("Closing tab");
+  BrowserTestUtils.removeTab(tab);
+});
+
+add_task(async function test_title_in_light_dom() {
+  let tab = await BrowserTestUtils.openNewForegroundTab(gBrowser);
+
+  info("Moving mouse out of the way.");
+  await EventUtils.synthesizeAndWaitNativeMouseMove(tab.linkedBrowser, 300, 300);
+
+  info("creating host");
+  await ContentTask.spawn(tab.linkedBrowser, {}, async function() {
+    let doc = content.document;
+    let host = doc.createElement("div");
+    host.title = "light";
+    doc.body.appendChild(host);
+    host.setAttribute("style", "position: absolute; top: 0; left: 0;");
+    var sr = host.attachShadow({ mode: "closed" });
+    sr.innerHTML = "<div style='width: 200px; height: 200px;'>shadow</div>";
+  });
+
+  let awaitTooltipOpen = new Promise(resolve => {
+    let tooltipId = Services.appinfo.browserTabsRemoteAutostart ?
+                      "remoteBrowserTooltip" :
+                      "aHTMLTooltip";
+    let tooltip = document.getElementById(tooltipId);
+    tooltip.addEventListener("popupshown", function(event) {
+      resolve(event.target);
+    }, {once: true});
+  });
+  info("Initial mouse move");
+  await EventUtils.synthesizeAndWaitNativeMouseMove(tab.linkedBrowser, 50, 5);
+  info("Waiting");
+  await new Promise(resolve => setTimeout(resolve, 400));
+  info("Second mouse move");
+  await EventUtils.synthesizeAndWaitNativeMouseMove(tab.linkedBrowser, 70, 5);
+  info("Waiting for tooltip to open");
+  let tooltip = await awaitTooltipOpen;
+
+  is(tooltip.getAttribute("label"), "light", "tooltip label should match expectation");
+
+  info("Closing tab");
+  BrowserTestUtils.removeTab(tab);
+});
+
+
+add_task(async function test_title_through_slot() {
+  let tab = await BrowserTestUtils.openNewForegroundTab(gBrowser);
+
+  info("Moving mouse out of the way.");
+  await EventUtils.synthesizeAndWaitNativeMouseMove(tab.linkedBrowser, 300, 300);
+
+  info("creating host");
+  await ContentTask.spawn(tab.linkedBrowser, {}, async function() {
+    let doc = content.document;
+    let host = doc.createElement("div");
+    host.title = "light";
+    host.innerHTML = "<div style='width: 200px; height: 200px;'>light</div>"
+    doc.body.appendChild(host);
+    host.setAttribute("style", "position: absolute; top: 0; left: 0;");
+    var sr = host.attachShadow({ mode: "closed" });
+    sr.innerHTML = "<div title='shadow' style='width: 200px; height: 200px;'><slot></slot></div>";
+  });
+
+  let awaitTooltipOpen = new Promise(resolve => {
+    let tooltipId = Services.appinfo.browserTabsRemoteAutostart ?
+                      "remoteBrowserTooltip" :
+                      "aHTMLTooltip";
+    let tooltip = document.getElementById(tooltipId);
+    tooltip.addEventListener("popupshown", function(event) {
+      resolve(event.target);
+    }, {once: true});
+  });
+  info("Initial mouse move");
+  await EventUtils.synthesizeAndWaitNativeMouseMove(tab.linkedBrowser, 50, 5);
+  info("Waiting");
+  await new Promise(resolve => setTimeout(resolve, 400));
+  info("Second mouse move");
+  await EventUtils.synthesizeAndWaitNativeMouseMove(tab.linkedBrowser, 70, 5);
+  info("Waiting for tooltip to open");
+  let tooltip = await awaitTooltipOpen;
+
+  is(tooltip.getAttribute("label"), "shadow", "tooltip label should match expectation");
+
+  info("Closing tab");
+  BrowserTestUtils.removeTab(tab);
+});