Is anyone else seeing a slow startup issue with Firefox 3.5? I'm talking about the initial load (after turning on your computer, for example). For me, startup times have increased dramatically with 3.5, and I've even tried creating a new profile. On my machine at home, cold startup takes between 30 and 60 seconds. On my laptop, it's between 15 and 30 seconds. Firefox 3.0.11 took on the order of 5 or 10 seconds on both machines.
I've got to believe something is wrong with 3.5. Maybe a 3.5.1 is on the horizon...
According to commenter Dean, cleaning out your temporary directory should help things (and it worked for me). To locate the temp folder, open a command prompt and type
echo %TEMP%. For me, the folder was under "Documents and Settings\Jonah\Local Settings\Temp." I cleaned this out using the Disk Cleanup utility (mine was 750 MB in size!) and startup was much faster (10 seconds or so).
I've found the requisite bug about this problem on Bugzilla: bug 501605. If I read this correctly, NSS uses files in the temporary folder to seed its random number generator. If you have tons of files, this operation takes a long time (the file scan operation isn't as efficient as it should be). There may also be other factors (as commenters in the bug report suggest), but this is at least one of the problem areas.