Yes, you can! Download and install them in separate directories. Then, launch each one individually with the -p flag to set up different profiles for each version (or at least one for testing). Then, after you have two separate profiles, create an icon for each on your desktop. Right click on the icon and select properties. In the 'target' field, add the following flags. c:\Programs\Firefox 56\firefox.exe -p Profile1 -no-remote c:\Programs\Firefox 57\firefox.exe -p Profile2