Sometimes, on Windows NT 4.0 (not sure whether this happens on other OSs), after shutting down Mozilla, the process will live on, still taking up memory, and also, it seems, CPU cycles. In fact, the CPU usage will stay at 100% until I kill the process. All of this is according to the Windows NT Task Manager. I've seen this happen several times recently, but don't yet have a way to reliably reproduce. I'll keep my eyes open and report back with the necessary steps, if I can figure out what they are.
I used to see this as well, but haven't seen this in the last few weeks. Casey, which build ID are you using?
I saw this behavior again yesterday, with the nightly from the previous day.
On Mozilla/5.0 (Windows; U; WinNT4.0; en-US; rv:0.9.1+) Gecko/20010608, running on NT4.0 server service pack 6A, closing mozilla close running proccess as expected, I'm going to mark it as worksforme, is worth of it making sure you have service pack 6A installed with a current mozilla build. Poster if the problem continues showing after that please feel free to reopen the bug.