Despite all the pain that we've to go through, most web developers have to live the fact that people use different browsers. So it's essential to be able to test our code against these different browsers.

This is the tools that I use to do so:

Multiple IE Version
Works based on the real IE dlls. Unfortunately doesn’t work on Vista.
https://tredosoft.com/Multiple_IE

Multiple Firefox version
portable firefox 3, 2 and 1 can be run as standalone apps without installation.
https://portableapps.com/apps/internet/firefox_portable