Using multiple versions of browsers (IE + FF)

Internet Explorer

I found this handy tool, IETester, for testing your websites in multiple versions of IE, each in a different tab. Currently it can test IE 5.5, 6, 7 and 8 beta.

Since it's an early alpha release there is a list of known bugs on their homepage but it is good enough to view pages in.

Firefox

I found the easiest way of testing in various versions of FF was to use a portable version since it doesn't need any installation and won't mess with whichever current install you have.

You can find all portable legacy versions if FF here.