Rated 4 out of 5 stars

Try newest version firetray-0.6.0 (released beta 12.10.2015)
I haven't problem yet.

This review is for a previous version of the add-on (0.5.6.1-signed). 

Rated 3 out of 5 stars

Freezenig thunderbird 38.3.0 on Linux (debian based) with firetray 0.5.6.1.

Edited: Use older addon. At my case version 0.5.3.x working better, but with any problems.

This review is for a previous version of the add-on (0.5.6.1-signed).