Has anyone found a solution to this issue? It's noted here on the Mozilla forums too.
The solution is to clear your temporary files (your temporary internet files, temp, etc. folders.) Apparently Firefox searches these folders at startup, which can cause it to be slow if they're crammed with files.
It will be fixed in an upcoming release.
For a more detailed guide on how to fix this, see: Workaround for Firefox 3.5 slow startups on Windows
Frans Bouma has a very good explanation of why this is happening
and the solution
In brief: Firefox is scanning your temporary files to create a random number for encryption. The solution, wait for them to patch in so that they rely on a windows function instead.
It's because 3.5 preloads temporary files (for random number generators, strangely) and other things (like fonts).
While it's not a direct solution disabling some addons (Firebug is my arch-enemy in terms of startup time) has been noted to speed up start-up. Common slow-downs are AdBlock, NoScript and Firebug.