Indeed there is a BUG (but you can workaround it). Rated 5 out of 5 stars

Update July 3rd:

Following my own advice, I have set the preferences to NOT export the All Mail folder through IMAP (I have created another folder to archive my old messages, so that it would behave like every other folder in every other IMAP implementation, instead of listing all my mail messages for all my folders like All Mail does).

Since then, this add-on is simply perfect.
Thank you again.

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This add-on works perfectly, but there is a bug:

If you set your GMail preferences to export the All Mail folder through IMAP, this add-on will mark the messages in that folder as read too, since it's in the [Gmail] folder.

But since the messages in that folder are just mirrors from the messages in other folders, marking a message as read in that folder will also mark the message as read in its original folder as well, INCLUDING THE INBOX.

Could you please fix this?
Thank you!

This review is for a previous version of the add-on (1.0). 

Little bug for me too Rated 3 out of 5 stars

Not everytime but almost 1/2, the inbox is entirely marked as read while launching Thunderbird.

Anyway, it's a good extension to keep focused on the essential.

This review is for a previous version of the add-on (1.0). 

Rated 1 out of 5 stars

It's a really interesting idea, but it marks as read all the messages also in the incoming folder, I miss some important email today, it's my opinion that this is a really big bug.

This review is for a previous version of the add-on (1.0). 

Usefull Rated 5 out of 5 stars

Does exactly what it sayes it does and that all it has to. Gmail is our work e-mail and this was a very irritating thing. Thanks to this extension its solved.

This review is for a previous version of the add-on (1.0).