QPWGraph works for me, but does not solve the issue. The author pointed on github, that the DE would be to blame, which highly unlikely as any other app in the tray does have icon and title. IMHO, the systemtray code is outdated. It will take some time before the author hopefully fixes this.
2.4.50 has an incomplete Patch and IS NOT SHIPABLE
F33 needs a Build for 2.4.51
Thunderbird 91.x packages are missing the GERMAN language files. A lot of different languages are available, but german is missing. This is a KNOWN bug, that SHOULD BE fixed in 91.1 according to Mozilla, but it isn't.