When using thunderbird addons like `FireTray` or `MinimizeTo Tray revived` then it should show the unread emails counter at the system tray like for example this image which shows 1 unread email (taken from cinnamon desktop): http://imgur.com/a/PbsdN. However on Plasma5 no matter how unread messages, the system tray icon doesn't change. Tested on Manjaro, Antergos and OpenSuse Tumbleweed. Every other desktop I've used works fine (xfce,mate,cinnamon even lxqt). Only Plasma5 has this problem, so I don't think it's related much to thunderbird, but mabye more to plasma's system tray. Reproducible: Always Steps to Reproduce: 1. Use `firetray` or `minimize to tray revived` addons on thunderbird. 2. Make sure there is an unread email. 3. If you can't understand the normal behavior, which is to show the unread emails number on the system tray icon, then try a different desktop environment to see the difference.
both of these tray extension aren't compatible with recent versions of Thunderbird - but i can reproduce absence of unread message count in system tray icon Plasma: 5.12.2 Apps: 17.12.2 Frameworks: 5.43.0 Qt: 5.10.1 Kernel: 4.14.20-2-MANJARO OS: Netrunner Rolling
I'm closing this as all Thunderbird tray extensions are not maintained or are not working with the latest version. Please reopen if there is working extension and is malfunctioning. In that case, please also check if exnesion is using XEmbed for tray icon. The easiest way is to kill xembedsniproxy, if icon disappears then XEmbed is used.