This is a very long-shot, but can you verify the path for Firefox 77.0.1 in the apparmor profile? I think Amber uses firefox-esr. Long ago, there was a similar issue with PureBrowser, and the suggestion was made to add the correct path/permissions.
If you are like me and download Firefox from Mozilla, I suspect the path to Firefox might be different than the profile. (I do not allow links to open from Thunderbird, so I do not see the problem.)