I noticed it too. When I upgraded to version 3.5, my Firefox took about an extra 15 seconds to startup. This is due to a bug in the security feature built into Firefox 3.5. The security feature involves scanning temporary folders looking for bits normally added by OS and other applications operations.
Firefox 3.5 scans more files and deeper into folders/subfolders which led to unexpected results where users with many temporary folders or files resulting in the slow start up.
The Fix
Applying the following fix will give a noticeable improvement to the start up time for Firefox 3.5. For this, we need to delete temporary Internet files and folders.
For Windows XP
Navigate to the following directories and delete the files.
C:\Documents and Settings\<username>\Local Settings\History
C:\Documents and Settings\<username>\Local Settings\Temporary Internet Files
C:\Documents and Settings\<username>\My Recent Documents
C:\Documents and Settings\<username>\Temp\
To delete temporary Internet folders, Start >> Control Panel >> Internet Options >> Under Browsing History, Click Delete >> Check all the options and click Delete. (See Screenshot below)
Windows Vista and Windows 7
C:\Users\<username>\Local Settings\History
C:\Users\<username>\Local Settings\Temporary Internet Files
C:\Users\<username>\My Recent Documents
C:\Users\<username>\Temp\
To delete temporary Internet folders, Start >> Type in Internet Options >> Under Browsing History, Click Delete >> Check all the options and click Delete. (See Screenshot below)