In the interest of browser-compatibility checking, and given limited resources, is it worthwhile to test my HTML 4/CSS 2 web applications and sites with older versions of Firefox, or is it sufficient to test with the current version (v3.6 as at the time of writing)?
If I need to test against older versions, why? What are the things that I need to look out for?
UPDATE 1: This applies to a generally available audience and we can assume the browser usage patterns are the same as for the general public.
As a general rule, test your application in as many different environments as you can. It’s never a BAD idea to test it against an older version of a browser.
Of course, given time limits and realistic requirements, you may want to limit the scope of your tests. For example, unless you’re creating an application for the government, you usually won’t need to test anything below Internet Explorer 6.
As for Firefox, the 2.x release was when it started to become really popular. It’s easy to imagine that there are many people out there still on Firefox 2.x. The main thing you need to watch out for is differences with the way CSS is handled. Older browsers may render certain CSS tags differently or just plain don’t have support for some of them.