You can have as many versions of Firefox as you want. However, each must have its own profile because of compatibility issues.

You can use sync to share passwords, bookmarks, and history. Anything else may not be compatible between different browser versions or computer systems.

Note: When installing, use the manual option and select the folder to install the program in. Examples; "C:\Program Files (x86)\Mozilla Firefox_54" "C:\Program Files\Mozilla Firefox_Current"

Note: After installing the program, it will ask if you want to start the browser. Answer NO ! ! !

If you do start the browser then, it will use whatever profile was last used by any Firefox. After installing, start Firefox with the -p option and create a new profile. Give the profile a name like: default-release Firefox_54

Once a profile is created, you will need to create shortcuts to call the different versions with their profile. Examples; "C:\Program Files (x86)\Mozilla Firefox_54\firefox.exe" -p Firefox_54 "C:\Program Files\Mozilla Firefox_Current\firefox.exe" -p default-release

Many thanks, Fred. Before I start messing about I'll need to do a full disc image to restore the status quo in case it all turns to custard. Most likely I'll need to refer back to you, if I may, so will leave the thread open for now. Appreciate your help.