There are also some things you can check manually. With Firefox open, go to "about
references" in the URL bar.
- Under general, the "Home Page" field will probably be this bad URL. Replace with "
https://google.com/" or some other safe default, or click "Restore to Default". You may also want to set "When firefox starts" to something safe such as "show a blank page" -- if it's set to "open previous windows" then this may be causing the bad page to come back.
- Then go to "about:addons" and then under "Plugins" and "Extensions", disable anything you don't explicitly recognize
If this does not work and the bad values come back (which is possible since Malware Bytes failed) then the infection may be deeper. I would definitely recommend the new profile option I sent earlier since this is safer way to reset all preferences and plugins in case some other malicious settings have been changed.
Finally if this does not work, you can try uninstalling Firefox/Chrome and re-installing. But the corrupted settings are probably in your username's preferences, not the app itself.
Finally, when you get this fixed I would definitely recommend adding an extension like "NoScript" for Firefox or equivalent for Chrome for general browsing. These take a bit more work to enable scripts for sites where scripts are needed (including macrumors, only for comments). But the benefit is much less risk from Javascript security bugs during general browsing.