First of all, you need to have an add-on called Memory Fox, which is an add-on the prevents Firefox from being such a memory hog. The add-on is experiment at the moment, however, once you register to get the add-on and install it, it will be reduce the amount of memory FF hog up.
Other ways to prevent Firefox from being such a hog is the following.
First, check your add-ons. Go to Tools, then add-ons and in the "Extension" tab uninstall or disable any add-on that you don't use. This includes Java Quick Starter and Microsoft .NET Framework Assistance.
Second, while in add-ons, go to the "Plugins" tab and disable any plugins that cause your browser to slow down. In particular these tree.
-Adobe Acrobat, which bring up PDF files from websites onto your browser. Turning it off will prevent this and your PDF files will open externally.
-get Plus Plus for Adobe. This is not needed.
- Java Platform.
-MetaStream
-Microsoft DRM
-QuickTime Plugin, which allows you to view QT video on websites
-Shockwave for Director
-Window Media Player Plug-in Dynamic Link Library
and finally, Window Presentation Foundation.
All of the above is not needed.
Third, you can tweak Firefox to run faster on your computer. The link below is video on who to do it.
http://www.youtube.com/watch?v=6hJAt_VasTc
Don't forget to restart Firefox in order for the changes to take affect.
Fourth, if you download files into your computer, go to Tool then Download and delete to files from the download list.
If you don't want to have any download file in the status bar in the future, go to Tools, then Option. Click on the "Privacy" icon and unchecked the box that says "Remember Download History."
Finally, always, always clean out your recent history, that way Firefox can work faster. Try doing every two weeks or every month, its up to you. If you don't how to clean out your browsing history, the video link below will show you how.
http://www.youtube.com/watch?v=HrR8CbmiZ-o
If you're browser is leaking memory, the link below will be very helpful with the problem.
http://windhair1000.blogspot.com/2009/03/how-to-fix-firefox-memory-leak.html
This measure works well with all versions.
I hope this information is very helpful.
Good luck