Is Restore Previous Session grayed out? That usually indicates that the previous session already was restored and you can try reopening closed windows and closed tabs from the History menu. Either:

• "3-bar" menu button > History > (individual window titles)
• (menu bar) History >Recently Closed Windows > (individual window titles)

If there's nothing there, could you check two other things:

Open your current Firefox settings (AKA Firefox profile) folder using either

• "3-bar" menu button > "?" button > Troubleshooting Information
• (menu bar) Help > Troubleshooting Information
• type or paste about:support in the address bar and press Enter

In the first table on the page, click the "Show Folder" button. This should launch a new window listing all your settings files.

Scroll down and copy all the file starting with sessionstore to a safe location for possible future recovery.

Does sessionstore.bak-datetime -- created during your last Firefox update -- look as though it might contain relevant data?

Do you find any extra files, such as sessionstore-1.js or other numbered files? If so, that indicates that Firefox cannot write to the main sessionstore.js file because it is locked or corrupted, and your windows/tabs are being saved in a numbered file. Unfortunately, Firefox can't actually restore any of those files automatically. In this situation, I believe you need to remove the sessionstore.js file (after exiting out of Firefox) and copy the latest numbered file to sessionstore.js.

If none of that pans out, you can check whether Firefox stored your sessionstore.js file in a restore point. Right-click the current file > Properties > Previous Versions. If you find something promising, make sure Firefox is closed before restoring it.

I was already starting to check an old backup (in favorites) and recent history to manually recover most of the tabs....but the last option you proposed worked perfectly!

I found a previous version (only 1 day ago) and recovered all my tabs.

Thank you very much!