[Users] can't get Firefox to open when I right click on small icon that shows text/html

[Gil Weber]

Hi, everyone.

I am using Claws-Mail 4.0.0 (flatpak) with 64 bit Linux Mint 20.2.
(I know, I know. Flatpak is not preferred.)

There is no more recent version of flatpak Claws-Mail available in the
Mint 20.2 repos. (At least I have not found one.)

I cannot get Firefox to open from an e-mail when I right click on the
small icon on the far right side of the window (that says: text/html).
I only get Chrome. 

When I right click and select: "open with" (no quotation marks) 
I get this:  xdg-open %s

I have tried to select other "open with" options including:

firefox %U
firefox '%s'
firefox --new-window '%s'

but they won't stick. The drop-down defaults back to:

xdg-open %s  and I get Chrome.

Can anyone please tell me how to get Firefox to open rather than
Chrome?

Thanks!   :o)
Gil