Found the blighter!
I've been plagued recently by the infamous "Yahoo Search has hijacked my search bar in Firefox" problem. Like a thousand other people, Googling for that search generally yields nothing concrete. Most of the answers show that responders simply haven't understood the problem - "just set your home page" - no it's the damn URL search bar thingy, not my home page you idiot, etc. etc. Anyway, someone gave me an idea - "Search for the yahoo.xml file and delete it, it's in C:\Users\ \AppData\Roaming\Mozilla\Firefox\Profiles\ .default\searchplugins\yahoo.xml" And there it was, so I deleted it and guess what? Nope, it didn't make a blind bit of difference. So I carried on looking, and there, in C:\Program Files (x86)\Mozilla Firefox\browser\searchplugins is the offending file. Shut Firefox and then delete the bugger. Job done. No, don't thank me, sponsor me... (Well, it worked for me. No hate-mail please ).