You may have ad/mal-ware. Further information can be found in this article; https://support.mozilla.org/en-US/kb/troubleshoot-firefox-issues-caused-malware?cache=no

Run most or all of the listed malware scanners. Each works differently. If one program misses something, another may pick it up.

The prefs.js file stores a lot of customizations, so if you remove it, Firefox will start a new one.

I'm not sure what Malwarebytes is finding in there. Perhaps it is preferences related to an extension that it removed earlier? But as long as the extension was removed, those preferences are just debris that can't do anything by themselves. Maybe check on the Malwarebytes forum.