Steps to reproduce:

I have a Laptop with AMD Vega 8 GPU and Ryzen 3500U.

I try to run Thunderbird Flatpak and Firefox Flatpak, both with an empty profile.

Actual results:

The start doesnt respond. It takes over 1min and seems to be completely normal, but the terminal shows "ATTENTION: default value of option mesa_glthread overridden by environment" three times. This also occurrs only after a few seconds.

Expected results:

start normally?

SYSTEM INFO
Fedora KDE with opensource mesa drivers
Flatpak Firefox from Flathub, latest

The Bugbug bot thinks this bug should belong to the 'Core::Widget: Gtk' component, and is moving the bug to that component. Please correct in case you think the bot is wrong.

Thanks for the report! Are you using Wayland?
Can the problem be avoided by starting Firefox with the following command (Wayland instead of Xwayland)?
$ MOZ_ENABLE_WAYLAND=1 flatpak run org.mozilla.firefox

I'm sure it's dupe of Bug 1787182.