Share and Enjoy, folks. If only all the wins were that easy.
Note: If you'd already upgraded to Firefox 8 and had LJlogin declared disabled before I made that updates.rdf change, then go into the Add-Ons Manager, click on the gear icon, and Check for Updates. Firefox should then allow you to enable LJlogin.
Edit 27 Dec 2011: Tested and updated to also be compatible with 9.x, which Mozilla surprise-released last week.
Further Edit 31 Jan 2012: Firefox 10 just came out. It has a new concept of extensions being Compatible By Default, so is now only really checks maxVersion if you specify strict compatibility (which I don't; I don't even know if that was a thing before now or if I'd have to add it) or under certain specialty situations I don't fall under. Just in case, I just bumped maxVersion in updates.rdf, but even without that, you should be able to keep on trucking. Share and Enjoy!