open tabs are stored in a file called session.json. Some software like CCLeaner routinely delete this file "for your privacy" with the result that it is not saved on shutdown. One thing to do is to turn on the message pane/reading pane (F8) it is turned off by default, so if it also disappears with the restart, that would indicate sessions.json issues.

Note that Thunderbird has to actually shut down, without a crash, for the file to be updated. You might want to check in the windows task manager that the Thunderbird process is actually ending. There have been instances, mostly involving antivirus programs, where Thunderbird simply hangs after the last window is closed so updates do not get written.