Suppose you would like to have firefox 3, 4 and now 5 simultaneously. Unfortunately renaming applications is not possible via Finder.
Suppose you have firefox3 installed. Do the following:
$ cd /Applications $ mv Firefox.app "Firefox 3.app"
Then install firefox4 and do that
$ mv Firefox.app "Firefox 4.app"
Install firefox5 and then:
$ mv Firefox.app "Firefox 5.app"
Now you would have all three versions:
$ ls -a | grep Firefox Firefox 3.app Firefox 4.app Firefox 5.app $
The even show up correctly in Applications tab:
(I messed up in renaming and Firefox 4 is actually Firefox4)
Note: unfortunately you cannot launch them simultaneously since Firefox has a mechanism to check if it is already running.