Actually you are the first one actually noticing this. The closed tab history is only kept for up to the *last* session but if you tend to close Firefox pretty often this, of course, may be a problem.

The reason for this is that I can't just rely on the Firefox built in sessions API and its closed tab history. If I do so, then, after some restarts, you will run into a bug that causes random tabs or even windows to restore as the internal IDs get reused which causes duplicated IDs. See here: https://bugzilla.mozilla.org/show_bug.cgi?id=1538119

To work around this problem, I move the Firefox internal closed tab history into a list in my Add-on to clear it. From this point I use a combination of my internal list (for tabs from the last session) and the Firefox built-in list (for tabs closed since browser start). This way I can be sure that no duplicated IDs will exist at any time and always the expected tabs or windows get restored. The only downside is that my internal list does not survive a browser restart. I could save it into storage but this could be a potential privacy problem for people who don't use session restore (and before you ask: Mozilla does not tell Add-ons about the session restore enabled status).

So currently with the existing bugs there are only two options:
a) Accept the small downside of only having last closed tabs for up to the last browser session
b) Randomly restore stuff you don't expect.

As most people don't even remember that they had interesting tabs open for more than one session into the past and random restores can be really annoying, I decided to go with a)