Yes.
Hot tip. You can enable 'Install extension from file' in mobile Firefox apps (Firefox, Fennec, Mull etc) by going to Settings > About Firefox (or About Fennec etc) > Tapping the name 5 times.
You should see a message about debug mode being enabled and the 'Install extention from file' option should be in the Advanced section of your brower settings.
Apparently it's a metadata bug, K9 shouldn't be listed as Thunderbird. See comments ITT.