, like all web browsers, needs to be regularly updated to keep up with security patches. Version 1.5 has an auto-update feature built-in, but unfortunately if you're not running as a local Administrator (at least in Windows), it doesn't work.
In one way, this is no different than Windows Update; if you go to the WU site as a non-admin, you get a message telling you to try again as an admin. The difference though is that whilst Tools -> Windows Update still works in IE if you are a non-admin (it politely asks you to try again with admin privileges), Firefox will grey out the Help -> Check for Updates menu item in this case. The inability to check for updates may lead a user to believe there are no updates available for their machine, which is clearly a bad thing because they will remain vulnerable to attacks. So, if you are using Firefox and you are not an Administrator, make sure to manually check for updates (or to boot Firefox as admin from time to time) in order to stay protected.
Something else to be wary of is that using the built-in update feature of Firefox will prompt you to re-start the browser so that it can finish updating itself. Unfortunately, if your next boot of the browser is as a non-admin, the browser will not be updated, but you won't be notified that it hasn't been updated, either. You must boot again as an Administrator for the update to take place.
Of course, personally I recommend people upgrade to Internet Explorer 7 Beta 2, but that's just me :-). Also note that at the time of writing, IE 7 is still an unsupported beta -- not production code -- so you should not install on mission-critical machines and read the release notes first.
Note 1: Obviously if you have installed software as an Administrator and placed it in the Program Files directory, a non-admin user will not have permissions to update the files. But that's not a problem; the problem is that non-admin users are given the impression that updates are not available (or perhaps not needed), leaving them with a false sense of security.
Note 2: Automatic Updates will still keep IE up-to-date for you even if you are not logged in as a local Administrator, so you should be sure to turn it on.