Open your file browser to your user account on the hard drive. Then do your search.

I did a search through my whole PC (Computer-> searchbar) Found nothing. (I searched for f.txt)

One place I've seen "f.txt" is when you are searching from Firefox's address bar or search bar with Google as your default search engine. Search engine suggestions are returned in a file named f.txt. However, under normal circumstances, Firefox intercepts that file and updates the suggestions list below the address/search bar, so you never see a download dialog. Perhaps there was an unusual delay and this file came in late, so Firefox assumed you requested it in a different way?

Thank you very much (both) for your response. This could be it, but I am not sure. If there is no download shown and I did not click to download something (the site is a "normal" site not a scam or anything like it) could it download anyway?

There is a thread on Reddit where people are speculating it is related to Google DoubleClick ads, but I don't see why it should have suddenly started. ??

https://www.reddit.com/r/firefox/comments/ixp3as/what_is_ftxt/

I read the reddit threat. I do not think it has something to do with Google. But with the pop up ads on the website. I think it is the same issue but with a different source for the problem. But I am glad to hear I am not the only one having the problem. Thank you again for all the help! I will mark this as solved because if it happens in the future I know what it is. Maybe a Update of Firefox brings a fix for this bug.

Press <Control> J to bring up the Downloads Manager.

I checked the download Manager, it did not download anything.