When developing a website it’s important to test it in as many different web browsers as possible.
If a client has a problem viewing the page there’s little point saying "but it works on my machine"
I have found that the differences between different versions of firefox to be small enough that testing in one version is sufficient.
Internet Explorer is so different between versions that each target version must be tested seperately.
There is a package for multiple IEs.
But you can’t then install the developer toolbar, and I’m not quite convinced that install is 100% the same as having a standard browser.
Fortunately Microsoft are aware of the problem and make a test virtual machine available.
I’ve used this for a while for testing IE6 - and it works very well. They have now released a verion of IE8 - time to check that my websites won’t break when users start updating.