I think I’ve found the solution (thanks to another forum, not my solution!):

“On my computer it looked like it was a problem made by a lavasoft ad-aware update. I removed 2 files: dsengine.cfg en dsengine.js. from the program files (x86) folder/Mozilla Firefox (root and folder defaults) , put the preferences like homepage back to my preferences and bing was gone en didn’t return. Regards, Peter” (https://support.mozilla.org/en-US/questions/1194334, 5th(-ish) post from the top)

It worked for me, so thanks to Peter! Wanted to share that here, too.

Much love!