Hi superlouuuu, I don't know what would explain that. Is the missing tab a peculiar address like a Firefox internal "about" page?

Does that tab have Back or Forward history (either arrow enabled) to get back to the lost page?

If you check the "Recently Closed Tabs" list in one of these places, is the tab available to restore from there:

• "3-bar" menu button > Library > History
• Library toolbar button > History
• (menu bar) History

If the problem is not a missing tab but an extra new tab from the current session sticking around after you restore your previous session from the menu (instead of automatically), that might be normal now.

Sometimes session data gets corrupted when you haven't visited a tab during the current session and something went wrong with restoring and saving the session.

Are you using "Restore Previous Session" as the startup setting or are you restoring the session manually via the "3-bar" Firefox menu button drop-down list or History menu?