For my recently former Windows brothers out there who may not be clued into this – you can’t run two instances of an app on OS X (at least not from the normal UI) nor can you install say, two versions of Firefox, side by side in the normal fashion.
This is obviously foreign to Windows users who are used to being able to do both of these things with ease, but especially for those of you who may be using Selenium (which doesn’t work with Firefox 3.5 yet), here’s a tip to get you started.
To install two versions of the same app side by side (in the applications folder) simply rename one. So, my firefox 3.5 install is named Firefox35.app and my Firefox 3 install is just Firefox.app. Now they can coexist.
To run two instances of the same app side-by-side (looking at you, World of Warcraft multi-boxers), just make a copy of the app and rename one. Since everything most apps need to run is self-contained in the .app structure, and there’s not a registry to deal with, that makes it easy.