Sounds like malware. It might not actually be in Firefox. It might be a program that's running separately and just messing with Firefox. The ''right way'' to troubleshoot it would involve the command line, running Firefox in safe-mode, selectively removing extensions, addons, plugins, checking running programs in task manager, and many more tiring monotonous steps... Just try removing your Firefox settings so you can start with a clean profile to see if that fixes it ... WARNING: If you do this, you'll also lose all of your bookmarks, extensions, plugins, themes, etc... Proceed at your own risk. On a Windows system, the files should be located somewhere like C:\Users\<your username>\AppData\Roaming\Mozilla. Close Firefox and remove that directory. When you open Firefox again, it's like starting from scratch.