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Any seasoned web developer is well aware of the issues with web standards compliance between the different version of Internet Explorer in the market place. A nice utility for this is Multiple IE, which allows multiple versions of internet explorer to be installed, or simply running IE 8 and enabling the developer tools which allows you to switch versions.

Firefox has now gone through several major versions and there are a few small differences between them, so the need to multiple copies has arisen here too. After a bit of googling it appears there is a solution, however most sites seem to have some of the instructions a bit wrong, so after a bit of fiddling, heres how to do it:

1. You've probably already installed a copy of firefox to the default installation location (C:\program files\Mozilla Firefox\"). Download and install the other versions you'd like to run, remembering to do a customised install, and specify a different install directory, say "C:\Program Files\Mozilla Firefox 3.0.15"
2. Now open the Firefox Profile manager: "Firefox.exe --Profilemanager" - best to do this from a command prompt, with one of your firefox installation directories.
3. Create a new profile, one for each separate firefox installation you have. Its probably a good idea to name the profile after the version of firefox it is for!
4. Then to launch your other versions of firefox, create a shortcut to firefox.exe, then modify the shortcuts properties, adding: -P “<your new profile name>″  to the end of the Target string (outside the quotes!) You will end up with something along the lines of: "C:\Program Files\Mozilla Firefox 3.0.15\firefox.exe" --P "ff 3.0.15" as the target, which uses a profile called ff 3.0.15
5. Its important to remember that you can only run one version of firefox at a time.