Tuesday was supposed to be Firefox 1.5's end-of-life farewell, with no more security or stability patches. But Mozilla has pushed back the date for ending support for the older version to mid-May.
If you haven't yet upgraded to version 2, you may as well get around to it before the now-delayed goodbye date. You're safer browsing with Firefox than with Internet Explorer 7 (IE7), but the open-source browser isn't invulnerable by any means. Mozilla just released a Firefox security patch about a month ago for both 1.5 and 2, and it's essential to keep your browser - whichever you might use - up to date.
What's more, version 2 doesn't introduce any radical changes that would take a lot of getting used to, but does have some nice new features like upgraded RSS feed support and built-in antiphishing.
See IE7 versus Firefox 2.