So, I don't know just how long this little bundle of joy has been around. I suspect long enough that I should feel like an idiot for not picking up on it before tonight. Anyway, the upshot of it is that since my testing indicates that Firefox 8 didn't change anything from 7 that I needed to care about, I could bump maxVersion in updates.rdf, and boom! Magic happens, everyone's browsers register the current version of LJlogin as compatible with FF8, everyone's happy. No new release necessary.
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!