--- a/layout/generic/nsFlexContainerFrame.cpp
+++ b/layout/generic/nsFlexContainerFrame.cpp
@@ -146,16 +146,20 @@ PhysicalCoordFromFlexRelativeCoord(nscoo
 // evaluate), these macros will ensure that only the expression for the correct
 // axis gets evaluated.
 #define GET_MAIN_COMPONENT(axisTracker_, width_, height_)  \
   (axisTracker_).IsMainAxisHorizontal() ? (width_) : (height_)
 
 #define GET_CROSS_COMPONENT(axisTracker_, width_, height_)  \
   (axisTracker_).IsCrossAxisHorizontal() ? (width_) : (height_)
 
+// Logical versions of helper-macros above:
+#define GET_MAIN_COMPONENT_LOGICAL(axisTracker_, isize_, bsize_)  \
+  (axisTracker_).IsRowOriented() ? (isize_) : (bsize_)
+
 // Encapsulates our flex container's main & cross axes.
 class MOZ_STACK_CLASS nsFlexContainerFrame::FlexboxAxisTracker {
 public:
   FlexboxAxisTracker(const nsStylePosition* aStylePosition,
                      const WritingMode& aWM);
 
   // Accessors:
   // XXXdholbert [BEGIN DEPRECATED]
@@ -180,16 +184,17 @@ public:
   }
   // Returns true if our cross axis is in the reverse direction of our
   // writing mode's corresponding axis. (From 'flex-wrap: *-reverse')
   bool IsCrossAxisReversed() const {
     return mIsCrossAxisReversed;
   }
 
   bool IsRowOriented() const { return mIsRowOriented; }
+  bool IsColumnOriented() const { return !mIsRowOriented; }
 
   nscoord GetMainComponent(const nsSize& aSize) const {
     return GET_MAIN_COMPONENT(*this, aSize.width, aSize.height);
   }
   int32_t GetMainComponent(const LayoutDeviceIntSize& aIntSize) const {
     return GET_MAIN_COMPONENT(*this, aIntSize.width, aIntSize.height);
   }
 
@@ -3096,30 +3101,31 @@ nsFlexContainerFrame::GenerateFlexLines(
   if (isSingleLine) {
     // Not wrapping. Set threshold to sentinel value that tells us not to wrap.
     wrapThreshold = NS_UNCONSTRAINEDSIZE;
   } else {
     // Wrapping! Set wrap threshold to flex container's content-box main-size.
     wrapThreshold = aContentBoxMainSize;
 
     // If the flex container doesn't have a definite content-box main-size
-    // (e.g. if we're 'height:auto'), make sure we at least wrap when we hit
-    // its max main-size.
+    // (e.g. if main axis is vertical & 'height' is 'auto'), make sure we at
+    // least wrap when we hit its max main-size.
     if (wrapThreshold == NS_UNCONSTRAINEDSIZE) {
       const nscoord flexContainerMaxMainSize =
-        GET_MAIN_COMPONENT(aAxisTracker,
-                           aReflowState.ComputedMaxWidth(),
-                           aReflowState.ComputedMaxHeight());
+        GET_MAIN_COMPONENT_LOGICAL(aAxisTracker,
+                                   aReflowState.ComputedMaxISize(),
+                                   aReflowState.ComputedMaxBSize());
 
       wrapThreshold = flexContainerMaxMainSize;
     }
 
-    // Also: if we're vertical and paginating, we may need to wrap sooner
-    // (before we run off the end of the page)
-    if (!aAxisTracker.IsMainAxisHorizontal() &&
+    // Also: if we're column-oriented and paginating in the block dimension,
+    // we may need to wrap to a new flex line sooner (before we grow past the
+    // available BSize, potentially running off the end of the page).
+    if (aAxisTracker.IsColumnOriented() &&
         aAvailableBSizeForContent != NS_UNCONSTRAINEDSIZE) {
       wrapThreshold = std::min(wrapThreshold, aAvailableBSizeForContent);
     }
   }
 
   // Tracks the index of the next strut, in aStruts (and when this hits
   // aStruts.Length(), that means there are no more struts):
   uint32_t nextStrutIdx = 0;