SpeedyFox is an excellent free app that allows you to defragment Firefox user profiles containing your bookmarks, history, cookies, passwords... in order to speed up the opening and execution of the browser.
Start by downloading the app SpeedyFox :
SpeedyFox for Windows (449.2) KiB, 1,610 downloads)
SpeedyFox for MacOS X (791.0) KiB, 813 downloads)
Then run it.
Select the profile to defragment and then simply click on Speed Up My Firefox! and wait a few seconds.

You must close Firefox before defragmentation.
Then click on Run Firefox and that’s it!
For testing this application under MacOS X Snow Leopard, the result is blatant: more than 10 seconds before to launch Firefox, almost instantaneous now!
An operation repeated from time to time to keep Firefox always at the top of its shape!
The result is obviously variable according to profiles, PCs, etc. since at my home on two positions I saw no difference and always the same slowness at startup.
I think this is especially true for those who use Firefox on a daily basis and install a lot of extensions, because so their profile is a real mess and this little utility makes a big cleaning up.
On the other hand, it doesn't reduce the start speed to a Chrome or Opera...
rotten! His walk not at all!
As DRIC says very well, the result varies according to the profiles. Mine weighing more than 50 MB (thousands of bookmarks, sites in history, very intensive use with dozens of tabs opened simultaneously and restored at every Firefox opening...) and more, being under MacOS (Firefox being poorly optimised under this system) there is a clear improvement.
There are several firefox extensions that do the job automatically, such as vacuum place improved:
https://addons.mozilla.org/fr/firefox/search/?q=vacuum&cat=all&x=0&y=0
@ Lamouche: Up your recommendation for this extension! Thank you.
it’s going much faster!
Thank you for making me know this little software. Your article is a year old, but I was quick to download the software and run it. No comparison: before “15 seconds”, after “nearly immedient”. Great! and thank you again.
Very effective, even if CCleaner has to do about the same thing with its “basic compacting” for Ffox.
Also supports Thunderbird and Skype among others.
In my case, the result for Skype is the most obvious: start time divided by three.
So very good recommendation.