IDK if it’s a proper forum to ask in since it’s pretty hard to find a communication channel for each particular Mozilla’s product.
Since around version 40, FF keeps periodically resetting my search engine settings, removing set keywords or even selectively deleting some of the added engines. I don’t know what causes this, but it’s annoying even though it happens rarely. Also I don’t know if it’s a known problem or if I should post on Bugzilla.
Firefox shouldn’t be the cause here.
Sadly, I think that there might be a variety of 3rd party tools that might be doing this. From anti-virus to malware.
SUMO has a good page on this, https://support.mozilla.org/t5/Problems-with-add-ons-plugins-or/What-is-search-hijacking/ta-p/29197, and there are a few tips on how to diagnose and fix things. If those fail you, I’d ask in the forum there again, probably got a few folks that don’t linger around here.
I wouldn’t think it’s some form of hijacking, mainly because I don’t get any erroneous results or unexpected pages, I just suddenly discover that some (usually not primary but one using a keyword) search engine has disappeared from the list. So I have to go to corresponding site and add that one engine again. Or, that engine is still there, but no keyword for it. Or, like the last time, after deleting 3 unused engines, I got other 3 added to the end of the list, even though all of them are of well-known sites.
Also, I have made antivirus checks several times since it first happened. I have NoScript and I check what I install.
To be sure, which forum do you mean, this one or some other?
The forum over at support.mozilla.org, https://support.mozilla.org/t5/Firefox/bd-p/Privacy-Security probably.