--- a/browser/extensions/formautofill/FormAutofillHeuristics.jsm
+++ b/browser/extensions/formautofill/FormAutofillHeuristics.jsm
@@ -234,17 +234,17 @@ this.LabelUtils = {
     _extractLabelStrings(element);
     return strings;
   },
 
   generateLabelMap(doc) {
     let mappedLabels = new Map();
     let unmappedLabels = [];
 
-    for (let label of doc.getElementsByTagName("label")) {
+    for (let label of doc.querySelectorAll("label")) {
       let id = label.htmlFor;
       if (!id) {
         let control = label.control;
         if (!control) {
           continue;
         }
         id = control.id;
       }