Small bug, otherwise working good:

Uncaught TypeError: idMatch.exec(...) is null
getVideoIDFromURL moz-extension://68085515-76f4-411b-a237-23c4cb9c1023/userscripts/Youtube HD.user.js?id=fc761042-a7d9-4c3e-95a4-16e8d4f586da:142
setResOnReady moz-extension://68085515-76f4-411b-a237-23c4cb9c1023/userscripts/Youtube HD.user.js?id=fc761042-a7d9-4c3e-95a4-16e8d4f586da:228
main moz-extension://68085515-76f4-411b-a237-23c4cb9c1023/userscripts/Youtube HD.user.js?id=fc761042-a7d9-4c3e-95a4-16e8d4f586da:379
xb https://www.youtube.com/s/desktop/18069be1/jsbin/webcomponents-sd.vflset/webcomponents-sd.js:40
dispatchEvent https://www.youtube.com/s/desktop/18069be1/jsbin/webcomponents-sd.vflset/webcomponents-sd.js:105
fire https://www.youtube.com/s/desktop/18069be1/jsbin/desktop_polymer.vflset/desktop_polymer.js:3844
d https://www.youtube.com/s/desktop/18069be1/jsbin/desktop_polymer.vflset/desktop_polymer.js:20057
31533_onFulfilled https://www.youtube.com/s/desktop/18069be1/jsbin/desktop_polymer.vflset/desktop_polymer.js:760
Jea https://www.youtube.com/s/desktop/18069be1/jsbin/desktop_polymer.vflset/desktop_polymer.js:770
Fea https://www.youtube.com/s/desktop/18069be1/jsbin/desktop_polymer.vflset/desktop_polymer.js:770
executeCallbacks_ https://www.youtube.com/s/desktop/18069be1/jsbin/desktop_polymer.vflset/desktop_polymer.js:769
fea https://www.youtube.com/s/desktop/18069be1/jsbin/desktop_polymer.vflset/desktop_polymer.js:660
eval line 4 > eval:142:20
getVideoIDFromURL moz-extension://68085515-76f4-411b-a237-23c4cb9c1023/userscripts/Youtube HD.user.js?id=fc761042-a7d9-4c3e-95a4-16e8d4f586da:142
setResOnReady moz-extension://68085515-76f4-411b-a237-23c4cb9c1023/userscripts/Youtube HD.user.js?id=fc761042-a7d9-4c3e-95a4-16e8d4f586da:228
main moz-extension://68085515-76f4-411b-a237-23c4cb9c1023/userscripts/Youtube HD.user.js?id=fc761042-a7d9-4c3e-95a4-16e8d4f586da:379
xb https://www.youtube.com/s/desktop/18069be1/jsbin/webcomponents-sd.vflset/webcomponents-sd.js:40
dispatchEvent https://www.youtube.com/s/desktop/18069be1/jsbin/webcomponents-sd.vflset/webcomponents-sd.js:105
fire https://www.youtube.com/s/desktop/18069be1/jsbin/desktop_polymer.vflset/desktop_polymer.js:3844
d https://www.youtube.com/s/desktop/18069be1/jsbin/desktop_polymer.vflset/desktop_polymer.js:20057
<anonym> self-hosted:1175
31533_onFulfilled https://www.youtube.com/s/desktop/18069be1/jsbin/desktop_polymer.vflset/desktop_polymer.js:760
Jea https://www.youtube.com/s/desktop/18069be1/jsbin/desktop_polymer.vflset/desktop_polymer.js:770
Fea https://www.youtube.com/s/desktop/18069be1/jsbin/desktop_polymer.vflset/desktop_polymer.js:770
executeCallbacks_ https://www.youtube.com/s/desktop/18069be1/jsbin/desktop_polymer.vflset/desktop_polymer.js:769
fea https://www.youtube.com/s/desktop/18069be1/jsbin/desktop_polymer.vflset/desktop_polymer.js:660

    // Get video ID from the currently loaded video (which might be different than currently loaded page)
function getVideoIDFromURL(ytPlayer)
{
const idMatch = /(?:v=)([\w\-]+)/;
let videoURL = ytPlayer.getVideoUrl();
let id = idMatch.exec(videoURL)[1] || "ERROR: idMatch failed; youtube changed something";
return id;
}

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Errors/Unexpected_type

Thanks for reporting. It looks like this occurs when searching, so it wouldn't actually affect functionality. But it should be fixed in the latest version now.