Browser speed is based on your isp and what site your connecting to. Also if you have too many Addons loading that can also affect the browser speed. And the other is if the AV is too aggressive or Firewall is blocking ports which could slow the internet. So as you can see there no one problem foe this and last it could be corrupted Firefox browser that can't function correctly and the only fix is to remove and delete the folder to fix the problem and reinstall. But if this the case one must backup their profile, logins, bookmark before doing this.