First, check your add-ins. If there’s some you don’t need, uninstall/disable them. See how much that improves speed.
If it’s still slow, download and install Opera, but leave Firefox installed until you test it. Opera operates a little differently and there’s a slight learning curve, but it’s worth it for the speed (it’s a much lighter browser without all the extra code). If you do install it, let me know and I’ll go over some finer points...
Any thoughts on Ubuntu?