What do you mean lost device?

Sorry about that lost device.

There is a limit on Synced Tabs - surely a lot less than 80 tabs; more like a dozen on Desktop / Laptop devices, but it may be less on "Mobile" Firefox devices.

Synced tabs expire automatically after 7 days.

There is a maximum to the payload of tabs (about 20 KB) that are synced, so if you have tabs with long titles and long URLs then you run quicker into this maximum.