FireFox
I have been running FF up through the versions (now at 6) and will likely switch browsers rather than "upgrade" to v7. No sooner than they released 7.0 and they reported a problem.
FF is too slow, a huge resource hog, often appears to "hang" but will eventually become responsive again. It seems to always be doing something in the background, even if I am just sitting at a page (could be any site). It is not very well thought out and nags to be upgraded and the most inopportune time, and has no place (that I've found) to do the upgrade manually when I am idle.
It doesn't matter if you open multiple tabs or multiple "instances", because if one window/tab hangs, they're all non-responsive. In the beginning I chalked it up to being new, but each "update" has only gotten worse. Additionally, there are features in other browsers that it does not have; and most of the "new features" are things I could care less about.