I helped my sister customize her browser last week and it worked well, so I think about sharing some tips here with you. I found them in several forums and blogs so, sorry I can't give a proper credits here
I tested it with Firefox 3.5 in Windows XP and Firefox 3.6.1 in Windows 7. I'll try to explain as simple as possible
Preparation: Back up your bookmarks folder
Go to menu Bookmarks>Organize Bookmarks...>[new windows pops up]>Import and export>Export html>Save your bookmark file
To restore your Bookmark folder, go through the same process but choose Import html and load the previous file you have saved
Close all Firefox windows
Clean up history
WARNING: This process may delete all the history, cookies info stored in your FF browser
Even if you use Clear recent history in Tools menu, all of the history are not deleted totally, and it makes FF start very slowly
There's a tip to clean up all the history and make FF boot faster
Go to Start menu>Run>[paste this: %AppData%\Mozilla\Firefox\Profiles]>OK
you'll see a folder with name like XXXXXXXXXX.default, enter it
Rename the file places.sqlite to places.sqlite.bak, exit the folder
Change history settings
Tools>Options>Privacy>choose Use customs settings for history
Instead of 90 days, choose the value that work for you, as for me around 10 days is enough>OK and Exit
Change cache and network settings
Those are the best setting for me, without changing the defaut setting to much that may cause problem in other systems and networks
Open a new windows, type about:config in address bar>Enter>Click the blue button to pass the warning message
After customizing, refresh the page to return to config page or just close the windows if you're done
Reduce the stored data in Back and Forward button/function. If you use multiple tabs at time, it will decrease the cache volume
In filter bar, look for browser.sessionhistory
you'll se browser.sessionhistory.max_entries, double click it and chang the value to 10
Increase the number of web loading requests to server, hence the website will load faster
Look for network.http.pipelining
double click network.http.pipelining and change value to true
do the same to network.http.pipelining.ssl
doublec click network.http.pipelining.maxrequests, chang the value to 30
Decrease loading delay time
Look for nglayout.initialpaint.delay, double click it and chang the value to 0
if there's not, you right click in blank space>New>Integer
Enter name: nglayout.initialpaint.delay, Enter value: 0