I can't use the latest on my Mac without upgrading the OS which I am not going to do until I get a new machine. The version I'm running now does everything I need it to so there is no need.
My only reason for the change was the PCWorld review which said that the latest Mozilla offering pretty much smoked the competition and was more secure than it used to be.
I have all the security updates for the version I run and stay away from porn sites and such which is where most viri spread from - that and emails.
I think they don't really have versions now, just updates which also patch security holes. They still give it a version number but overall, it is not a major version, just a update in an attempt to keep up with Chrome which has given Firefox and IE some stiff competition. I am rooting for Firefox, but Chrome is very impressive and is constantly gaining market share which means as a web developer it can't be ignored and isn't going away anytime soon. Actually, Opera is a very good browser too, but since they aren't open source I haven't really used it much except for testing sites.