Ive been using Mozilla firefox and Thunderbird for quite some time now, and im very happy with them. I especially like the tabbed browsing capabilities of firefox, allowing you to open several tabs at once instead of new windows, awesome feature..
Ive found that some site simply wont work with mozzilla though, primarily those with heavy flash integration or media viewers, and i often have to reinstall plug ins as they mysteriously stop working..this may have been addressed since i got my copies , which are almost a year old now..i havent bothered to check, as its not that big a deal.. i keep ie on standby for such occasions..
BTW: since i started using mozilla stuff, ive havent gotten a single virus, and only rarely do i get infected with spyware, i suspect it happens when i use the ie browser though..
thumbs up for Mozilla from me!