If your Firefox Portable does not show up as an option for default browser in Widows 10 then you can use a program found here to register it: Register Firefox Portable with Default Programs in Windows
If you find that Thunderbird or other programs still do not recognize Firefox Portable as the default web browser then try one of these:
1. Close Firefox Portable and restart.
2. Create a test.html file and open it while Firefox Portable is closed. Windows should call on Firefox Portable to open it.