In the meantime I have reproduced the issue as originally reported, by updating firefox to 67.0+build2-0ubuntu0.18.04.1 in a bionic VM, launching it, then dist-upgrading to cosmic and launching firefox again. I have also verified that deleting the file compatibility.ini in the profile folder "fixes" the issue.
Packages are being rebuilt, sequentially.
In the meantime I have reproduced the issue as originally reported, by updating firefox to 67.0+build2- 0ubuntu0. 18.04.1 in a bionic VM, launching it, then dist-upgrading to cosmic and launching firefox again. I have also verified that deleting the file compatibility.ini in the profile folder "fixes" the issue.