Just to confirm, do you have your startup setting on

"When Firefox starts: Show your windows and tabs from last time?"

See: Startup, home page, tabs, and download settings.

If no: Could you check the History menu for "Restore Previous Session" and see whether it works?

If yes: Could you check the History menu for recently closed windows that you could re-open?

There are several reasons that Firefox might not restore all your windows and tabs:

(1) Private windows are not restored. If Firefox is set to start up automatically in private mode -- either

• "Firefox will: Never remember history"
• "Firefox will: Use custom settings for history" + "Always use private browsing mode"

-- then no session history is saved to disk.

(2) Firefox is set to clear history when it closes. This article describes where to find that setting and make sure you don't clear history at shutdown: Delete browsing, search and download history on Firefox -- see the "How do I make Firefox clear my history automatically?" section.

(3) External software cleans out your browser data. For example, if you use CCleaner, make sure it is not set to clear Firefox session data.

Or if you do not want to restore windows and tabs automatically, make sure cookies are not cleared because if Firefox can't produce those cookies for those sites, the sites definitely will require you to log in again.