hello, you may have malicious addons or adware present on your system. please perform all these steps:

1. reset firefox (this will keep your bookmarks and passwords)
2. afterwards go to the firefox menu ≡ > addons > extensions and in case there are still extensions listed there, disable them.
3. finally run a full scan of your system with different security tools like the free version of malwarebytes and adwcleaner to make sure that adware isn't present in other places of your system as well.

Troubleshoot Firefox issues caused by malware

I used malwarebytes and it found 115 problems that 2 different anti-virus programs did not find. After deleting these I had no problems with Firefox. The malware had buried itself in the registry and so could not be deleted in the programs area which I had tried. I'm really disappointed in the anti-virus programs out there. They seem to miss a lot of the malware that buries itself. I am very careful in what I download. Thanks for your help.

Anti-virus software is different than anti-malware software - you need both these days. I don't know what the "fine line" that differentiates to two types of applications is, but it seems that malware gets into a PC via the user installing seemingly innocuous programs that carry the malware which then infects the PC thru the Windows Registry. Using the "sandbox" feature that is available in many Security Suites these days might help to prevent malware intrusion. But that's a subject for a more appropriate fora.