fire fox is more secure.. I also have IE 7 wich is much similar to firefox.. but i prefer t use firefox for secutirity reasons.. it is much safer.. everyone should try it and decide for them selves..
At the time or writing this in order of favour 1) IE7 and Opera 2) Firefox 3) IE6 and netscape Unfortunately i have to use all of them, ensuring my sites work across as many as possible.. ps. the biggest pain in the ass to code for is IE however it produces the nicest results (the pages and fonts in IE7 look 10x better than either opera or firefox)