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Computer Hijacked AWAY from Firefox??

I guess this is an incredibly naive inquiry, but.. can anyone tell me please how I can maintain Firefox as my default search engine? 

I appear to have been hijacked (?) in that my PC continually defaults to Yahoo as my home page, etc. which I don't like or want.

 I'm not sure what's going on- whether we're talking a virus or malware or what but it's annoying and having used Firefox in the past, that's what I would prefer to stick with.

Can anyone please advise how I can default to Firefox? Thanks in advance.
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