When does it change back? Is it when re-downloading Firefox, or does your current installation of Firefox not save your choice?

m_2 said

I need that Firefox respect the search engine I have decided. I don´t like Tapufind!

Hello m_2,

Please, don't blame Firefox - unfortunately you seem to have picked up some malware, cause TapuFind is a browser hijacker ....

See : https://www.pcrisk.com/removal-guides/14248-search-tapufind-com-redirect-mac

Go to the Firefox menu (≡) => Preferences => Add-ons => Extensions and remove anything that looks unfamiliar or suspicious.

Next step : run malwarescans - 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. They all work differently - what one program doesn't pick up, the other might.

Especially for Mac : https://www.malwarebytes.com/mac/

Yes, Thank you very much for your help!

m_2 said

Yes, Thank you very much for your help!

My pleasure - I'm glad that you got rid of that nuisance !

You inadvertently marked your own post as Chosen Solution ....

You can rectify it by clicking on the "Undo" button to the right of your post, and then mark my (first) post by clicking on the "Solved the problem" button to the right.

Thank you in advance !