Are you experiencing this pause on Windows, Mac OS, or Linux? I ask because my employer has some crap McAfee security theatre software installed (Windows) that causes this same issue for me for any application that loads any large-ish file (or large number of files) on startup. Since Firefox is going to load a bazillion things from cache on startup it (or similar software) could be causing the problem.

I had support temporarily disable McAfee's "on-access scanner" thing so I could test it and the problem went away immediately. Everything was nice and snappy again!

Of course, they turned it back on after the test...