Should have asked about this sooner, as it's something I've seen since
the very first F38 upgrade. Not sure if this is Fedora or Thunderbird issue.
When Thunderbird opens, it does so postage-stamp size, way up in upper
left hand corner of window, so small I almost missed it at first.
Right-clicking "maximize" restores it to full screen display.
You didn't say which desktop environment you're using. If you're using kde, you can set properties on the app to start maximized. On other desktop environments there's probably a similar setting, but I don't know it.