Sometimes a problem with Firefox may be a result of malware installed on your computer, that you may not be aware of.

You can try these free programs to scan for malware, which work with your existing antivirus software:

• Microsoft Safety Scanner
• MalwareBytes' Anti-Malware
• Anti-Rootkit Utility - TDSSKiller
• AdwCleaner (for more info, see this alternate AdwCleaner download page)
• Hitman Pro
• ESET Online Scanner

Microsoft Security Essentials is a good permanent antivirus for Windows 7/Vista/XP if you don't already have one. Windows 8 has antivirus built-in already.

Further information can be found in the Troubleshoot Firefox issues caused by malware article.

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This was my first thought as well but I have scanned my system twice with Windows Defender. This problem only started occurring when I upgraded to windows 10 and installed Firefox again. In case there is no specific solution, can you let me know how i can block specific websites in Firefox. Also, when I uninstall Mozilla, is it a clean uninstall or are there some files still remaining behind? If some files do remain behind, is there any way to perform a clean uninstall?

BTW, where did you download Firefox from? Hopefully to got it from an official Mozilla server or webpage. Many "downloads" portals package popular programs like Firefox inside their own installer and include "extras" like PUP - Potentially Unwanted Programs, or just plain Malware.

OK, Windows Defender didn't find anything, Use the other anti-malware programs I posted in the order of their efficacy. Not every anti-malware application will find everything, most of the time it takes 2 or 3 to get rid of everything.

To completely remove Firefox, you need to also remove the Profile folder. https://support.mozilla.org/en-US/kb/profiles-where-firefox-stores-user-data#firefox:win10:fx40