Step 1: Start Firefox and export your bookmarks as a file on your hard-drive (we'll need them later).
Step 2: Type firefox.exe - P in the Run box of Windows. (see screenshot)
Step 3: Click the Create Profile button without making any modifications to your existing profile (which is normally called "default")
Now when you Start Firefox in the new profile, you are very likely to be impressed with the speed. You can import the bookmarks that you saved in Step 1. If you have made any changes to the Firefox Dictionary, copy the persdict.dat word list file from the old profile folder to the new one.
Yes, there won't be any old Firefox add-ons in the new profile but the browser will be extremely quick and won't hog the CPU - just the way you want Firefox to run on your computer.
And if you ever need to revert to the old profile, just type Firefox -P again and click the old profile. Nothing is lost.