One thing to keep in mind is that when comparing security vunerabilities is the criticality of the problem.
For instance Mozilla does things like bug bounties and such. Since it's open source the amount of people able to look and prod around the code is much higher. With IE there may be flaws but they won't be commented on unless they are actually exploitable.
The proof is in the pudding when you look at the type of vunerabilities. Firefox may have dozens of smaller and more insignficant flaws found.. this is going to be normal for the rapid state of developement and relative youth of the browser.
IE on the other hand tends to get much more crippling and severe flaws due to it's (relatively) ancient code base.
Both projects are huge. Mozilla project has millions and millions of lines of code that they go thru....
Mozilla project isn't just about a browser. It's a entire web-based (or not) application developement platform.
People have used it to build numerious different browsers other then Mozilla or Firefox, they've used it for IDE's, custom intraweb applications, ftp/email/calender clients, word proccessors and all sorts of other stuff.
There is even a experimental Linux distro were they've used Mozilla technology and javascript programming to build a entire User Interface designed specificly for simple users and koisk stuff.
If your curious what that looks like here is a screenshot:
http://www.symphonyos.com/screenshots/beta1-desklets-ss.png
http://www.symphonyos.com/
(they have a live cd aviable via bittorrent if you want to try it out)
For firefox vs explorer keep this in mind:
(from secunia)
22 advisories for Firefox 1.x, out of those 14% are unpatched and the highest unpatched critial level is rated 'less critical' (level 2 out of 5). Out of the total of 22 advisories 23% are rated 'highly critical'.
http://secunia.com/product/4227/
69 advisories for IE 6.x from 2003-2005. Out of those 28% are unpatched with the highest unpatched threat level is rated 'highly critical' (level 4 out of 5). Out of the total of 69 advisories 14% are 'Extremely Critical' compared to Firefox which has NO level 5 vunerabilities. 29% of those 69 advisories rate highly critical.
So there you have it.
With IE 6.x since 2003 you have about 30 vunerabilities that rate at or above the highest level of threat level of any vunerability that Firefox has aviable, according to secunia. In comparision firefox has a total of FIVE vunerabilites that got the 'highly critical' rating.
_all_ of those have patches aviable.. unlike IE 6.x.
IF your using Windows XP with SP2 installed you can be reasonably safe using IE. Many of those vunerabilities are mitigated by some of SP2's special security features... HOWEVER if your using Windows 2000, Windows ME, Windows 98, or any other Windows OS (such as server editions) it is VERY bad policy to use IE on them to browse the internet more then is absolutely nessicary. Microsoft does not care about maintaining application security in anything other then it's latest OS and latest service pack level, apparently. Which is perfectly fine for MS if they want to do that. Stuff costs money. With Firefox, since it's OS independant, you can be sure to get most of the latest security measures/patches aviable irregardless of your platform.