Firefox 3.5 pulls data from certain file locations for randomised security purposes. Sometimes, though, that can lead to annoying slow start-ups in Windows. You can speed things up by cleaning out certain folder locations.
For whatever reason, having too many temporary, history or recent document files sitting in your Windows locations slows down Firefox 3.5's data generation processes, as has been reported as a bug. The Mozilla Links blog recommends cleaning out these folders to move the browser's work along:
C:Documents and Settings*user*Local SettingsHistory C:Documents and Settings*user*Local SettingsTemporary Internet Files C:Documents and Settings*user*My Recent Documents C:Documents and Settings*user*Temp
On Vista or Windows 7 systems, simply replace "Documents and Settings" with "Users." Setting up Firefox to automatically clean out your temporary files after browsing, as explained at the link below, helps prevent the accumulation from building again, but users who trade browsing time with Internet Explorer may have to occasionally work this manual method to wipe out those files.
Workaround for Firefox 3.5 slow startups on Windows [Mozilla Links]