What version do you have? Version 3.6.10 is the latest.
Is it only FF that is slow, or your machine as well?
Are you using FF in a virtualized or Safe Mode as this may slow things down?
How many add-on's do you have in FF? More than 5 may slow you down as well.
Do you clean up your machine after browsing? Many of us use
CCleaner. It is a freeware system optimization, privacy and cleaning tool. There is a
Slim version available as well at
http://www.piriform.com/ccleaner/builds - 4th option down. It removes unused files (cache, temporary Internet files, etc.) from your system - allowing Windows to run faster and freeing up valuable hard disk space. It also cleans traces of your online activities such as your Internet history. Additionally it contains a fully featured registry cleaner.