Two versions of Firefox

First post, newbie.
I have Firefox ESR 52 installed on my desktop (for old school reasons, eg Classic Theme Restorer), and would like to try latest Firefox Portable ESR. I would install it to the same 8 GB flashdrive that has my mail client: Thunderbird Portable Legacy 3.1 (great little program, clean functional interface, never any issues). There's over 7 GB of free space on this flashdrive.
Couple of concerns:
Any way to set Firefox Portable to be the default browser which launches when I click on a link in an email in Thunderbird Portable?
Will there be any potential conflicts with my Firefox ESR 52, the default browser installed on the desktop? I want to keep ESR 52 unaffected by the new Firefox Portable.
OS: Win 7 Pro (64 bit)
Thanks