Try booting firefox in safe mode (there should be an option to do so under All Programs > Mozilla Firefox.
This will boot firefox in safe mode - not using any extensions or extra themes.
If it runs fast(er) on safe mode, try uninstalling some of your extensions - or, as Richard says, disabling them one at a time.
If you find the pages aren't loading as fast as you would like it to - try using this little trick.
Quote:
1: Type "about:config" into your address bar and wait for it to load.
2: Alter the following entries:
change "network.http.pipelining" to "true", "network.http.proxy.pipelining" to "true" and "network.http.pipelining.maxrequests" to 30.
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This will enable your pipelining for http requests, proxy requests and it will change the max requests for the pipelining to 30.
Hope this helped.