Rated 3 out of 5 stars

@Schtritoff: I have discovered that your solution works as long as I don't close my Firefox browser too often. There is a fix for this when it happens, just install the GM NG installation file again into your Firefox and close your browser and launch it again. It works, at least for me. Just remember, closing the browser (from time to time) damages the addon's installation and you will have to install the GM NG addon again.

Rated 5 out of 5 stars

@Schtritoff: Thanks for your advice. I can confirm it really helps, now all my gapps accounts work flawlessly again.

Rated 3 out of 5 stars

GM NG stopped working with the newest (paid) version of Google Apps (tm) accounts today. Older google apps emails (non-paid accounts) and generic gmail.com emails work all right. Looks like another fix is needed here...