I had been happily auto-hiding the Firefox navigation bar on my small-screen netbook for a couple of years already, until that add-on (LessChromeHD from the Prospector series) stopped working after having upgraded to Firefox 35. So I started Firefox from the command-line and spotted this error:
addons.xpi WARN Error loading bootstrap.js for lessChrome.HD@prospector.labs.mozilla: TypeError: redeclaration of variable event (resource://gre/modules/addons/XPIProvider.jsm -> jar:file:///home/frank/.mozilla/firefox/jy6bws91.default/extensions/lessChrome.HD@prospector.labs.mozilla.xpi!/bootstrap.js:226:8)
A quick look at the source code confirmed “event” was declared twice, once on line 210 and a second time on line 226. The fix, obviously, is simple; on lines 226-228 replace all references to “event” with e.g. “shownEvent”;
let shownEvent = document.createEvent("Event");
shownEvent.initEvent("LessChromeShown", true, false);
You can do this yourself by unzipping “lessChrome.HD@prospector.labs.mozilla.xpi” in your extenstions-folder, editing boostrap.js and updating the xpi. Or you could wait for the Mozillians to update LessChromeHD.