Pinned tabs are stores along with session data in sessionstore.jsonlz4 in the Firefox profile folder. so you could possibly restore them if you have an older copy of this file.

You can try to restore a recent upgrade.jsonlz4-<build_id> in the sessionstore-backups folder when available or use a utility to browse Windows System Restore points to see if you can find a recent sessionstore.jsonlz4 file.

You can use the button on the "Help -> More Troubleshooting Information" (about:support) page to go to the current Firefox profile folder or use the about:profiles page (Root directory).

• Help -> More Troubleshooting Information -> Profile Folder/Directory:
  Windows: Open Folder; Linux: Open Directory; Mac: Show in Finder
• https://support.mozilla.org/en-US/kb/profiles-where-firefox-stores-user-data

You will normally find these files in the sessionstore-backups folder:

• previous.jsonlz4 (cleanBackup: copy of sessionstore.jsonlz4 from previous session that was loaded successfully)
• recovery.jsonlz4 (latest version of sessionstore.jsonlz4 written during runtime)
• recovery.baklz4 (previous version of sessionstore.jsonlz4 written during runtime)
• upgrade.jsonlz4-<build_id> (backup created during an upgrade of Firefox)

You can copy a file from the sessionstore-backups folder to the main profile (i.e. one level up) and rename the file to sessionstore.jsonlz4 to replace the current file with Firefox closed.

• make sure to backup the current sessionstore.jsonlz4

You can look at this tool to inspect a compressed jsonlz4 sessionstore file. This tool works locally, no uploading done.

• https://www.jeffersonscher.com/res/scrounger.html

One possible cause of this is closing out Firefox by "X"ing individual windows. Firefox may only restore the last window you close before it quit, rather than what is most important.

Could you check whether either of these are available (not hidden or grayed out):

• "3-bar" menu button > History > Restore Previous Session
• (menu bar) History > Restore Previous Session

If they are missing or grayed, check either:

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

By default, Firefox stores up to three closed windows.