The basics are still the same for over 13 years.

Separate folders for each Firefox version and a separate Profile for each. Best not to share the same Profile between a Release and Nightly.

https://support.mozilla.org/en-US/kb/profile-manager-create-and-remove-firefox-profiles

Note if you let Nightly update it gets checkins almost everyday so it gets updates each of those days. The version will seem to stay the same.

You can use say about:buildconfig in Location (address) bar to see the changes and this about: url can be found on about:about

So you know the channels Release < Beta < Aurora (developers edition) < Nightly

Installing multiple versions on Mac OS X is basically dragging the Firefox application out of the DMG file to the desktop and rename the folder. You can drag the renamed Firefox application folder to the Applications folder like you would normally install Firefox.

You need to use the terminal to create a new profile for that version. Make sure that no profile is selected as the default to make the Profile Manager show when you start Firefox. You can check that in the profiles.ini file.

• http://kb.mozillazine.org/profiles.ini_file
• http://kb.mozillazine.org/Profile_Manager

• http://www.businessinsider.com/how-to-run-firefox-4-and-3-simultaneously-in-mac-os-x-with-multiple-firefox-profiles-2011-3

Hello, the reason I'm asking this question is because usually? i can't have the same app installed twice on my Mac but i guess they are different and can have both?

First, why do you even want to use the Nightly? As James pointed out, Nightly gets updates almost every day, and it's not always stable - it is an alpha1 level program and not intended for the "average user". If you are dependent upon certain add-ons, they may not always work in Nightly - your add-ons may need to be updated to remain compatible with Firefox / Nightly and with as often as Nightly is updated many add-on developers wait until a lot closer to the release of a new Firefox version before spending their time updating their add-on. ("Release" is like 12 to 18 weeks after Nightly.)

And as cor-el mentioned, the OSX environment isn't very conducive to having two separate versions of the same application at the same time; with Linux and Windows operating systems it is possible to actually switch between two (or more) separate version installations AND even have them all running simultaneously, if you know the "trick".