Never mind, I figured it out.
It is VERY annoying to me that all of the recent (since December 2025) automatic Firefox upgrades have deleted my previous session tabs, with no "Restore previous session" option offered. Prior to December, every Firefox upgrade was transparent with no effect on my open tabs. This new behavior annoys me so much that I'm likely to switch to another browser if there isn't a solution to stop this from happening. Please tell me how to avoid this problem, or consider fixing it in the next upgrade.
Modified