For those who have lost tabs, I discovered that selecting Exit from the hamburger menu instead of X (close) avoided loss of tabs. Also, if X is clicked by mistake, I found that restarting my laptop (or possibly just closing Firefox and reopening it, I'm not sure) restored the tabs I had open.

When first opening Firefox, a succession of several sessions, one with only 1 tab and others with a group of tabs, rapidly appear before displaying just one session with the right group of tabs. I'm now going to disable this add-on and stick with Firefox's "open previous windows and tabs" option.

Edit: The behaviour above (2nd paragraph) still happens even after disabling the add-on, both with X (close) and Exit. AND this edit paragraph is missing (despite definitely clicking Update review) if closing with X!!