Pale Moon, originally forked from Firefox many years ago (although the codebases have diverged so far that most Firefox patches no longer apply). Still xul, still supports Firefox extensions from back in the day as well as extensions purpose-written for it. On the downside, it occasionally isn’t compatible with the latest bleeding-edge nonstandard Javascript features—I keep Vivaldi around for the extremely rare occasion when something goes wrong with a site that I absolutely must visit for some reason (I think I’ve needed it twice in the past five years).