There will always be bugs in software so don't expect that you're completely safe just because you switched from IE to Mozilla. Safer you are but not to the point that Mozilla is bugs-free. However, they do issue pathces considerably faster (don't have anything to back that up since I'm not an IE user but that's what I've heard from IE users). That goes to users of all browsers. Most of the vulnerabilities probably wouldn't exist if there weren't the users who practice "unsafe" browsing...