This would only happen if the use "Clear history when Firefox closes" to clear the browsing history. Clearing the history that way will prevent Firefox from opening the previous session to protect your privacy. You will have to clear the history manually to avoid this. Clearing history on close down shouldn't be necessary to fix issues. You can do that once.

Did you check "History -> Recently Closed Tabs/Windows"?

You would have to restore a copy of sessionstore.js from the Time Machine or other available backup to recover these lost tabs.

You can use this button to go to the current Firefox profile folder:

• Help -> Troubleshooting Information -> Profile Directory:
  Windows: Show Folder; Linux: Open Directory; Mac: Show in Finder
• http://kb.mozillazine.org/Profile_folder_-_Firefox

Make sure to close the Firefox application with all windows still open.

• Mac: Firefox -> Quit Firefox

Thank you for providing this service! I did not use "Clear history when Firefox closes" to clear the browsing history. I do not see that option on my version of Firefox at all. There must be some other reason why the tabs were cleared from the history. I used “Clear all History” and as I mentioned before my setting is: Firefox - Preferences - When Firefox starts: “Show my window and tabs from last time.” I am not trying to recover the lost tabs. I followed your suggestion and checked "History -> Recently Closed Tabs/Windows" and they are not backed up. I just want to know how to set up Firefox to prevent this from happening again. Is there another menu selection or box that I need to check or uncheck so that Firefox will “Show my window and tabs from last time” after I choose “Clear all history”?