I’m using FF9, and that’s as far as I plan to go. I have the executables for 3.6, I’m tempted to reinstall it and forget all this 9, 10, 12 garbage.
I'd suggest you not. If you're using FF-9 you are most of the way there as the newest version (FF-12 released today) will assume all your add-ons work so if they are working in FF-9 they will work in FF-12.
My problem is that the upgrade from 3.6 to a more recent version of FF will take out my entire screen arrangement and break links for any add-ons not supported by a later version (and I have a few). Also on the downside, today, April 24, is the last day FF will support v.3.6. Any security breaches after today will not be addressed so that version (and earlier) can be hacked.
The next version, FF-13, due out in June will drop versions of Windows earlier than XP with service pack 2. So its a mixed bag, but I'm just going to have to bite the bullet and upgrade in the next few days.