That happens to me when I shut down my PC without closing Firefox myself
Firefox
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Yeah, I have this setting disabled and it works as intended for me (only pinned tabs are re-opened). Maybe OP has an extension that is somehow triggering a Firefox crash on shut down?
For me it's really after closing the last window. If I open a new one, it will re-open and load every tab that was in the last closed window.
A few months ago, there was an issue on plasma wayland on regarding apps not exiting properly, which triggered this for me.
Regardless, the problem you're describing of screams of Firefox not terminating gracefully. Can you tell us a bit more about your setup
Hmm... Maybe it's my session files that are corrupt? Maybe I should remove my profile from my local machine and recreate it.