After installing Firefox 123.0 I ran a Mullvad Connection check https://mullvad.net/en/check and found I had a DNS leak. Turns out that Firefox had changed the settings under “DNS over HTTPS” so that my Mullvad DNS wasn’t being used any more. After changing this setting to “off” in Firefox, the Mullvad check passed.
My Firefox DNS over HTTPS settings are as they were before the upgrade. Maybe this is Flatpak-specific?