Firefoxes is a shell scripts that will install all major versions of Firefox on OS X, and set up separate profiles so you can use them simultaneously. Currently it installs:
- Firefox 2.0.0.20
- Firefox 3.0.19
- Firefox 3.5.9
- Firefox 3.6.28
- Firefox 4.0.1
- Firefox 5.0.1
- Firefox 6.0.1
- Firefox 7.0.1
- Firefox 8.0.1
- Firefox 9.0.1
- Firefox 10.0.2
- Firefox 11.0
- Firefox Beta
- Firefox Aurora
- Firefox Nightly
- Firefox UX Nightly
You can set it up so it only installs the specific versions you want.
Optionally, the script can install Firebug for each version of Firefox too.
It will also set icons that contain the version number:
http://gkoberger.net/images/notepad/firefoxes.png
For Safari, check out Multi-Safari.
For Chrome/Chromium, install any version you want, change the app name from Chromium.app to e.g. Chromium 19.app for clarity, then disable auto-updates for that version.