Quote Originally Posted by Lemur
There's no need to uninstall Firefox to run Opera; to the best of my knowledge, you can have both running on the same machine with no conflicts.

Opera has two distinct advantages over Firefox in terms of security:
  1. It is closed source, which would normally be a disadvantage, except that:
  2. Opera has a smaller market share than IE and FF. This means that it is a smaller target for spyware and malware creators.

I have seen no studies of real-world security differences between Firefox and Opera, but both of the above points are valid. Personally, I find Firefox plenty secure.
Firefox's open source nature wouldn't be a problem if it was patched/updated much more frequently than it is. The problem with Firefox is that it's now getting much more popular. Back in the old days of Mozilla 1.5/1.6 and Firebird and early Firefox the browser was so little known that you didn't really have to worry about it.