You need to be careful, but it can be done.

Each Firefox must be placed in its own Programs Folder. Also, each must have its own profile.

You can use sync to share the profile data, BUT different versions may not be able to share certain settings, add-ons . . . . So sync only passwords and bookmarks.

Download the full installers for each version you want to use. When you run the installer, select the Manual option and place each in its own programs folder (see pictures).

Note: When the installer is done, Do Not run the browser yet.

Note: Whatever version is loaded Last will be the default. Also, create the profiles before you start installing other Firefox's. +++++++++++++++++++++++++++++++++++++++++++++++ With Windows, each Firefox must have its own batch file program to send the nessassary information into the browser when started.

Here is an example (on one line only); C:\Program Files (x86)\Mozilla Firefox Current\firefox.exe -no-remote -P Firefox_v_57

https://developer.mozilla.org/en-US/docs/Mozilla/Command_Line_Options

Usage: C:\PROGRA~1\MOZILL~1\firefox.exe [ options ... ] [URL]
      where options include:
 
 -h or -help        Print this message.
 -v or -version     Print Firefox version.
 -P <profile>       Start with <profile>.
 -migration         Start with migration wizard.
 -ProfileManager    Start with ProfileManager.
 -no-remote         Do not accept or send remote commands; implies -new-instance.
 -new-instance      Open new instance, not a new window in running instance. (Does not work on Windows) 
 -UILocale <locale> Start with <locale> resources as UI Locale.
 -safe-mode         Disables extensions and themes for this session.
 -console           Start Firefox with a debugging console.
 -jsconsole         Open the Error console.
 -browser           Open a browser window.
 -new-window  <url> Open <url> in a new window.
 -new-tab     <url> Open <url> in a new tab.
 -preferences       Open Options dialog.
 -search     <term> Search <term> with your default search engine.
 -setDefaultBrowser Set this app as the default browser.

(see pictures)

.....

Current Nightly builds (still 68, but will soon go to 69) should use its own profile when you aren't specifying or creating a profile with the Profile Manager and thus shouldn't interfere with a release. On Windows they will probably also install in their own Firefox program folder.

See also;

• Bug 1474285 - Implement profile-per-install [67]

(please do not comment in bug reports
https://bugzilla.mozilla.org/page.cgi?id=etiquette.html)