I've tried the 64 bit version of Pale Moon several years ago and it didn't seem as performant as Firefox (could've been due to the setup and environment; I didn't bother to investigate deeper). The fact that many XUL extensions have been abandoned or have stopped development is a concern though. Thanks for the Basilisk mention. It may be a good alternative for older machines that the current Firefox doesn't support.
It's basically XUL Firefox w/o WebExtension crap (they played with WebExtensions and bailed).
Made by the same people who make Pale Moon, it's basically a successor.
The main downside, of course, is the XUL extension authors are unlikely to keep updating their extensions now the XUL is dead on OG Firefox.