To run different versions of Firefox at the same time takes some prior setup. I am going to assume first that you have the release version of firefox installed. To mean: you have (or should first) installed the version of firefox from getfirefox.com.
Opening Firefox Profile Manager
With firefox installed, yet not running, open up your terminal application.
For Windows/*nix systems:
In the profile manager: make 3 additional profiles and name them
"aurora", "beta", and "nightly"
Uncheck the box that says
"Use the selected profile without asking at startup"
Hopefully your profile manager window looks something like this.
Downloading Other Firefoxes
First let’s proceed to mozilla.org/beta. From here you want to download and then install the Aurora and Beta versions of Firefox.
Now go to nightly.mozilla.org and download and install the nightly version of Firefox.
Running all Firefoxes
If you correctly unchecked the ‘use selected profile at startup’ box in your Firefox profile manager, then when you run any version of Firefox the profile manager should pop up. If not, then see the section “Opening Firefox Profile Manager.” Now you want to select the appropriate profile for the version of Firefox you’re running.
And that’s it! You can now open all 4 versions of Firefox at the same time! Feel free to claim your badge here!