Hi Ulid,
applications installed through Flatpak can be run with flatpak itself.
First you have to check the internal flatpak name for Thunderbird; `flatpak list`
will give you the list of all you flatpak applications, you have to check for
`org.mozilla.Thunderbird`.
Then, you should be able to launch Thunderbird from the command line using
`flatpak run org.mozilla.Thunderbird`.
As you cannot use parameters in dependencies Konfigurator, you should create
a bash file (eg: somewhere in you home folder or wherever you like) whit the
previous command.
/usr/local/bin/thunderbird.sh
--- STARTS HERE ---
#!/bin/bash
flatpak run org.mozilla.Thunderbird
--- ENDS HERE ---
And then set that script in Konfigurator. I'm assuming that you have bash installed
and your distro has /usr/local/bin; any path will be fine too; paths outside you home
directory could require root permissions to create that file.
Hope that helps
Davide