hello JoaBar, there might be malicious software present on your pc. please try these steps:

1. reset firefox (this will keep your bookmarks and passwords)
2. afterwards go to firefox > 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

You got Feven extension or similar. You need to clean your extension list or scan with anti-malware program. Also, many of your ad-ons look suspicious. E.g. Mindspark is known platform for adwares, ≈

_{Link to removal guide at "www.2-viruses" website (where Giedrius_M is the admin) removed by moderator. This link is considered advertising spam and promotion for Enigma Software's questionable SpyHunter product. (ref. 1 ... ref. 2)}

Giedrius_M: I removed the link in the text "Feven extension" from your reply since it's considered advertising spam. See Mozilla Support rules and guidelines.