Thanks Wesley! I agree with the comment here that this should no longer be the case on Desktop thanks to Bug 1573758, and we would expect synced tabs to be kept around much longer than a week.
I just checked and it looks like the same change was not applied to Firefox for iOS, on which the tabs still expire after 21 days. I've filed an issue here to track that.
I don't know what the expiry is for tabs saved from Firefox for Android, but I wouldn't be surprised to find that it's limited to a few weeks.
It may also be that a user's synced tabs disappear or are hidden for reasons other than deliberate expiry - for example if their device gets disconnected from their Firefox Account then I believe our clients will ignore any synced tabs stored by the now-unknown device. or if they hit one of the several reasons for being moved to a new storage node.