I use Mozilla for 95% of my browsing. It's excellent. The only snag is that it uses "proper"HTML and doesn't implement the non-standard MS HTML as used in IE.
So it won't work on some websites. Mostly, I just don't visit those. For the couple that are non-standard and that I do need, I keep a copy of MSIE uniquely for them.
I've never had a virus or a nasty infect my PC through Mozilla.