Try to clean your PC, uninstall any unneeded softwares, also if there are more files in the disk it will slow your PC, what is the size of those files that you moved to the external HDD? Try to use google chrome if you have it, like Ztruker said what are the add-ons in your firefox? If the entire PC is slowing down try to clean the fans, heatsinks, RAM, graphics card, processor, PSU, motherboard, and harddrive with compressed air. If you want to remove/touch some parts be sure to touch any metal that is connected to earth so it won't discharge any static electricity in the parts that you are trying to remove because it may short it and cause it to die.