I've used Firefox for 3-4 years now, but I think I'm getting to the end of my relationship with it as well.
About a week ago (pretty much coinciding with the release of 3.6.2), I noticed that my PC started running slowly and a check of the CPU usage showed one of the cores was maxing out. Task Manager identified Firefox as the offender, closing it and re-opening it sorted the problem instantly.
However, it wasn't a permanent fix, it started recurring every few hours. I thought it might have been add-ons (I'd just added a couple) so I stripped out the new stuff, but that didn't make any difference.
Worst issue, when it does it, is that it disables all of my menus. I can't click on anything, including the little red box to close it down.
This afternoon, I did a full delete of Firefox, cookies and all, then went to Mozilla.com and downloaded the latest. 2 hours later, and Core2 is maxed out with Firefox taking 35% of my CPU load.
I'd been an Opera addict for several years before switching to Firefox, looks like I'm going back there again.