Have you got a lot of extensions? If Yes, maybe some of them are working in the background that you're not aware of.
Open your Task Manager (Ctrl+Shift+Esc) and take a look at how much memory Firefox is using under 'Processes'.
You could also try opening Firefox in Safe Mode and make a comparison from the Task Manager.
Have you got other programs working in the background aslo? Basically, anything working in the background will slow Firefox, and anything else.
I didn't think many people bothered with Tweaking Firefox anymore, obviously I'm wrong! Anyway,
here's a link to a good up to date tweaking guide.