Do I want to do this? What are the pros and cons?
Well, my 3.6.x Firefox was becoming unstable (sometimes it would "semi-freeze", that is the Firefox window would be fully functional, but I couldn't close it without going to Windows Task Manager) so I bit the bullet and upgraded to the 12.x.
"It's different from what I'm used to, so it can't be any good."
Actually, the differences are minor and I can live with them. But why do they have to change things that didn't need changing? The biggest annoyance (and it is totally trivial) is that they moved the "Bookmarks" icon from the left side of the screen over to the far right. I still look for it in the wrong place. Tabs that are currently active are moved to the top of the screen above the URL, instead of below the tab bar icons. Those are about the only user interface changes I have noticed. No doubt there are plenty of others in areas of the browser that I don't use.
All in all, the changes are benign. Lucky for Mozilla... I was all prepared to write them a scathing letter if I had to. THAT
would have sure taught them a lesson!