Current versions of Firefox, Chrome, ans Safari are Evergreen Web Browsers - they automatically update on startup.
Internet Explorer is not.
Evergreen Web Browsers, however, can give headaches to IT administrators. Some web applications or websites might work only with a specific version of a web browser and break in a newer version. Evergreen Web Browsers are also annoying when making a presentation. Basically, Evergreen Browsers are not for everyone all the time.
Correction: October 11, 2013
It appears that Internet Explorer 10 now comes with a feature that keeps it auto updated making it evergreen as well.