You may want to check your Firefox and make sure it is fully updated. Then try running it without add-ons loaded. If it works better then, you should allow them one at a time to see which one is causing the conflict.
If this doesn't help, try some good deep scans for malware, viruses, etc., to make sure nothing is on the computer.
You also may want to check to see what is loading when you start up, there may be something running in the background that really doesn't need to be.