How can I use multiple browser versions at one time?
Firefox

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:



Safari

For Safari, check out Multi-Safari.

Chrome/Chromium

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.

Opera

Download old Opera versions here: ftp://ftp.opera.com/pub/opera/mac/

IE

Get VirtualBox and then run this ievms script. It will automatically download legal Windows images for testing purposes and set up virtual machines for every IE version you need.