diff --git a/src/content/peerconnection/dtmf/js/main.js b/src/content/peerconnection/dtmf/js/main.js
index 89ba841d7..e4de08cea 100644
--- a/src/content/peerconnection/dtmf/js/main.js
+++ b/src/content/peerconnection/dtmf/js/main.js
@@ -173,7 +173,9 @@ function dtmfOnToneChange(tone) {
 }
 
 function sendTones(tones) {
-  if (dtmfSender && dtmfSender.canInsertDTMF) {
+  // firefox doesn't implement canInsertDTMF, so assume it's always available
+  // Ref: https://bugzilla.mozilla.org/show_bug.cgi?id=1623193
+  if (dtmfSender && (typeof (dtmfSender.canInsertDTMF) === 'undefined' || dtmfSender.canInsertDTMF)) {
     const duration = durationInput.value;
     const gap = gapInput.value;
     console.log('Tones, duration, gap: ', tones, duration, gap);
@@ -193,4 +195,4 @@ function addDialPadHandlers() {
   }
 }
 
-main();
\ No newline at end of file
+main();