How to make FirefoxPortable.exe the default browser

I recently configured a new laptop with a dual boot system, Win 7 Pro and Win 10 Ent. I have a third partition that each can see and that is where my portable app platform is installed. When I click on the shortcut FirefoxPortable.exe everything works fine from both OS's. However, the first time I opened the app a Mozilla page came up and asked if I wanted to make Firefox my default browser. Of course I said sure. Now, if I click the FirefoxPortable.exe shortcut everything still works fine. However, the problem that occurs, is when I click a link from a non Firefox source. A new profile has clearly been created as the first time I did this in each OS it asked to copy settings from wherever. I said no and moved on.

I do NOT have a local copy of Firefox installed anywhere else on either OS and a Mozilla profile folder has been created in my local %USERPROFILE%\AppData folder on the C: (system) drive.

If you can tell me how to make FirefoxPortable.exe the default browser or another way to always use the same profile all of the time I would appreciate it.

Thanks!