GPGShutdown complains about non-portable Firefox

I have Portable Thunderbird (60.9.1) installed with GPG Portable (2.2.19) installed into the ...\App\gpg\ subfolder of Thunderbird Portable.

This is somewhat recent, perhaps the last month or so.

Dialog looks like:

=======================================
GPG Shutdown | PortableApps.com [X]
--------------------------------------------------------------------

C:\Program Files (x86\Mozilla Firefox\firefox.exe

[ O K ]
=======================================

I've also had it just say "0" in place of the Firefox path.

I should note that Firefox is not portable Firefox, it's the standard version, which is running.

I've tried a fresh install of Thunderbird Portable + GPG Portable (with migration of the pre-existing Thunderbird profile), and it still happens.

I'd love to keep this from popping up.

Thanks.