I'm not really sure about which is the most secure.. but I'm prety sure that IE7 is the mos unsecure.. lol..
Anyways, Opera is not used all too much so I guess not too many people will be making hax and exploits for it, whilst FF is the webs leading browser for now... and more people will try to look for security holes in it..
Simply just like phpBB, got too famous, too many people used it, too many people tried to hack phpBB forums, and that's what happened... now phpBB is somewhere in the bottom when it comes to security...

my 2 cents
