can't click on Firefox e-mail links and open the Claws-Mail compose window

Hi, everyone.

I am as certain as I can be that when I was using Mint 19.0 with
Claws-Mail 3.X I could click on an e-mail link in Firefox and it would
open the Claws-Mail compose window, automatically inserting the intended
addressee's e-mail address. (At least I think I remember this
happening.)

Now I am using Mint 20.2 with Claws-Mail 4.0.0 (flatpak). When I click
on an e-mail link in Firefox 97.0.1 the main Claws-Mail screen opens,
not the compose window. 

I have the following written in my notes from years past for setting up
Firefox email links in the about:config screen.

network.protocol-handler.expose.mailto		boolean true
nework.protocol-handler.external.mailto	boolean true
network.protocol-handler.app.mailto	claws-mail  ("string")

Is anyone able to click on an e-mail link in Firefox and get it to open
the Claws-Mail compose window with the address inserted?

Thank you.