I just had a go-around with my eldest son over his loading software onto my iMac. He put Firefox and Skype on it without my knowledge or permission. I read him the riot act over this (he has screwed up my computers before, and I'm not letting him ruin this one).
However... neither Skype nor Firefox are malicious and I realize that Firefox is one program I wouldn't mind using. I've been using Safari as my browser, and I haven't had any problems with it crashing totally, though it has hung up a few times. Do you use Firefox as a browser on your computer? For that matter, Thunderbird?