Speedyfox is tool which can increase the speed of your Firefox by defragmenting the profile database.
Boost Firefox startup in up to 3 times!
Speed-up browsing history
Quicker operation with cookies
Available for both Windows and Mac OS
Read more and download
Just download the Speedyfox, run it (make sure you have closed the Firefox)
Select the profile and click "Speed Up My Firefox!" that's all.
This really improved the startup time of my Firefox.
[Via MegaLeecher]