For every security problem with FF there is about 10 more for IE.
And as far as speed goes, I don't think there is a difference, though IE may render poorly coded pages better (lots of people out there don't code up to standards).
Mozilla works long and hard releasing builds for Firefox each and every day, as well as hourly.. MS has nowhere near that dedication.. IE7 has copied so much from Firefox, not just features, but It copied layout, icons and other stuff.. If you look at a lot of MS products and services they are copied from something, that is why they are always getting sued by companies because of it.
Just look at www.live.com it is a copy of google customize, right down to the links in the right top corner (settings, logout)... They hide it the best they can, but simply they copy and duplicate other successful products, and fail at it!
BTW, If IE was open source, you would be hearing about so many security issues that you would be horrified to use it... Firefox security issues are easier for security companies and people to identify because the code is right there for them to look at.