New mail indicator envelope for Thunderbird in Ubuntu 18.04

I miss one little detail from my old 16.04 that I have not been able to find or activate in 18.04: a mail envelope in the top bar that turns blue when there are new unread mails in Thunderbird. How can I install or activate this feature in Ubuntu 18.04 Unity 64-bit? I installed Birdtray as suggested here previously, but it does not do what is required.

It seems like Birdtray is the best (and only) option you have got, unfortunately.

You did find an answer to this before at

There is a gnome extension

But not sure it works on newer versions as have stopped using thunderbird myself so cannot test it

My apologies, for some reason this answer did not reach me. Thanks. Does not seem to work on Ubuntu 18.04. I will have to fiddle with the Birdtray settings to see if it can do the job.