I would recommend you install a antivirus like Malwarebytes. Run it and see if anything improves. If not you can also try CCleaner, although you need to know that if you run it it is going to clean out your pc (the junk, incl your browsing history and saved website passwords etc). Hope this helps

Also check your browser addons and extentions. Google the name of each one to see if you can remove them. Ive also seen exe shortcut manipulation. It might also be the case here. Right click on your firefox shortcut and make sure it only executes the firefox.exe.