Independent research.
Opera has the least security holes. Firefox and IE are about even (i believe IE has more serious holes, but Firefox has more overal flaws from the article i read a couple of months back)
Look up webbrowser benchmarks and FireFox usually ranks pretty low on page fetches and rendering.
As for system resources. On a PC system Firefox takes longer to start than IE, uses more memory than IE and i've had it crash on a stable Windows system more times than i care to remember.
Don't get me wrong, Firefox is still better than IE imo, but then Opera is better than Firefox.
At the end of the day though, now browser is safe (especially when given to the average novice home user) so firewalls, virus scanners and so forth, is highly recommended.
I'd say the best advice is use the browser that best suits your needs and skills, and stick as much security software on your internet system as you can (and exercise some common sense. Don't download files called "i love you.exe" hehe)