Rated 3 out of 5 stars

Please excuse the poor packaging.

Anyway, I've just created version 0.6.5.2.1 and packaged a new xpi. It works for me, so let me know if you have problems.

https://github.com/twang817/firefox-gmail-manager-ng/raw/0.6.5.2.1/dist/gmail_manager_ng-0.6.5.2.1.xpi

I did not release under 0.6.5.4 because it appears that there is yet-to-be-released code in the original author's master that is currently broken. So instead, I added another dot release to the 0.6.5.2 version (which appears to be what is here on addons.firefox.com).