Resetting Firefox as told in https://support.mozilla.org/en-US/kb/refresh-firefox-reset-add-ons-and-settings often works. If it doesn't, there's more to try.
I get a pop up that goes something like, your computer may be infected, click here for a quick 10 second scan. In the background is Norton Antivirus. I also don't have Norton. Obviously phising. My browser is Firefox. Can anyone tell me how I can remove this malware? Thanks.