od rsjtdrjgfuzkfg (Vývojář) v Led. 23, 2020 · trvalý odkaz

It works for me on both Windows and Linux (including after restarts). If the issue only occurs after some time, the issue might be a varying order in which add-ons are loaded (especially if you use other add-ons influencing Thunderbird's main window). It thus might be able to fix it by disabling the conflicting add-on (and/or telling me which add-on is in conflict through the email listed as support contact).