--- a/browser/extensions/formautofill/FormAutofillHandler.jsm
+++ b/browser/extensions/formautofill/FormAutofillHandler.jsm
@@ -257,23 +257,44 @@ class FormAutofillSection {
       if (!element) {
         continue;
       }
 
       let maxLength = element.maxLength;
       if (
         maxLength === undefined ||
         maxLength < 0 ||
-        profile[key].length <= maxLength
+        profile[key].toString().length <= maxLength
       ) {
         continue;
       }
 
       if (maxLength) {
-        profile[key] = profile[key].substr(0, maxLength);
+        switch (typeof profile[key]) {
+          case "string":
+            profile[key] = profile[key].substr(0, maxLength);
+            break;
+          case "number":
+            // There's no way to truncate a number smaller than a
+            // single digit.
+            if (maxLength < 1) {
+              maxLength = 1;
+            }
+            // The only numbers we store are expiration month/year,
+            // and if they truncate, we want the final digits, not
+            // the initial ones.
+            profile[key] = profile[key] % Math.pow(10, maxLength);
+            break;
+          default:
+            log.warn(
+              "adaptFieldMaxLength: Don't know how to truncate",
+              typeof profile[key],
+              profile[key]
+            );
+        }
       } else {
         delete profile[key];
       }
     }
   }
 
   getAdaptedProfiles(originalProfiles) {
     for (let profile of originalProfiles) {

--- a/browser/extensions/formautofill/test/unit/test_getAdaptedProfiles.js
+++ b/browser/extensions/formautofill/test/unit/test_getAdaptedProfiles.js
@@ -1059,16 +1059,90 @@ const TESTCASES = [
   },
   {
     description: "Use placeholder to adjust cc-exp format [mm - - yyyy].",
     document: `<form><input autocomplete="cc-number">
                <input placeholder="mm - - yyyy" autocomplete="cc-exp"></form>`,
     profileData: [Object.assign({}, DEFAULT_CREDITCARD_RECORD)],
     expectedResult: [Object.assign({}, DEFAULT_CREDITCARD_RECORD)],
   },
+  {
+    description: "Test maxlength=2 on numeric fields.",
+    document: `<form>
+                 <input autocomplete="cc-number">
+                 <input autocomplete="cc-exp-month" maxlength="2">
+                 <input autocomplete="cc-exp-year" maxlength="2">
+               </form>`,
+    profileData: [Object.assign({}, DEFAULT_CREDITCARD_RECORD)],
+    expectedResult: [
+      Object.assign({}, DEFAULT_CREDITCARD_RECORD, {
+        "cc-exp-year": 25,
+      }),
+    ],
+  },
+  {
+    description: "Test maxlength=4 on numeric fields.",
+    document: `<form>
+                 <input autocomplete="cc-number">
+                 <input autocomplete="cc-exp-month" maxlength="4">
+                 <input autocomplete="cc-exp-year" maxlength="4">
+               </form>`,
+    profileData: [Object.assign({}, DEFAULT_CREDITCARD_RECORD)],
+    expectedResult: [Object.assign({}, DEFAULT_CREDITCARD_RECORD)],
+  },
+  {
+    description: "Test maxlength=1 on numeric fields.",
+    document: `<form>
+                 <input autocomplete="cc-number">
+                 <input autocomplete="cc-exp-month" maxlength="1">
+                 <input autocomplete="cc-exp-year" maxlength="1">
+               </form>`,
+    profileData: [Object.assign({}, DEFAULT_CREDITCARD_RECORD)],
+    expectedResult: [
+      Object.assign({}, DEFAULT_CREDITCARD_RECORD, {
+        "cc-exp-year": 5,
+      }),
+    ],
+  },
+  {
+    description: "Test maxlength=0 on numeric fields.",
+    document: `<form>
+                 <input autocomplete="cc-number">
+                 <input autocomplete="cc-exp-month" maxlength="0">
+                 <input autocomplete="cc-exp-year" maxlength="0">
+               </form>`,
+    profileData: [Object.assign({}, DEFAULT_CREDITCARD_RECORD)],
+    expectedResult: [
+      {
+        guid: DEFAULT_CREDITCARD_RECORD.guid,
+        "cc-exp": DEFAULT_CREDITCARD_RECORD["cc-exp"],
+      },
+    ],
+  },
+  {
+    // It appears that negative values do not get propagated.
+    description: "Test maxlength=-2 on numeric fields.",
+    document: `<form>
+                 <input autocomplete="cc-number">
+                 <input autocomplete="cc-exp-month" maxlength="-2">
+                 <input autocomplete="cc-exp-year" maxlength="-2">
+               </form>`,
+    profileData: [Object.assign({}, DEFAULT_CREDITCARD_RECORD)],
+    expectedResult: [Object.assign({}, DEFAULT_CREDITCARD_RECORD)],
+  },
+  {
+    description: "Test maxlength=10 on numeric fields.",
+    document: `<form>
+                 <input autocomplete="cc-number">
+                 <input autocomplete="cc-exp-month" maxlength="10">
+                 <input autocomplete="cc-exp-year" maxlength="10">
+               </form>`,
+    profileData: [Object.assign({}, DEFAULT_CREDITCARD_RECORD)],
+    expectedResult: [Object.assign({}, DEFAULT_CREDITCARD_RECORD)],
+  },
 ];
 
 for (let testcase of TESTCASES) {
   add_task(async function() {
     info("Starting testcase: " + testcase.description);
 
     let doc = MockDocument.createTestDocument(
       "http://localhost:8080/test/",