If you’ve recently been gettings hangs with Mozilla, you may be running into bug 146884 — in a nutshell, if you turn on HTTP Pipelining (Preferences -> Advanced -> HTTP Networking), Mozilla will likely hang, sucking upwards of 100% CPU within a few minutes.
It’s been suggested that the bug could be added as a dependency of the “make mozilla 1.1beta not suck” and “make mozilla 1.1 not suck” bugs. But, those requests have fallen on deaf ears.
So, in the meantime, just be sure to turn off HTTP Pipelining.
PS For the Mozilla nuts in the audience, I see that Sspitzer has all kinds of cool pre-made Bugzilla queries on his homepage, such as “smoketest blocker bugs”.