It happens, Firefox crashes. Fortunately most of the time it will automatically restore all of your open tabs for you. Unfortunately, sometimes it comes up after a crash without restoring your open tabs or offering you an option to do so. If you’ve numerous windows with many tabs open this can hurt.
Fortunately there is an easy fix. In the address bar simply type in this address:
about:sessionrestore
This should cause the restore session dialogue to appear and you are good to go.
WARNING: If, before doing this, you’ve already opened new tabs, whatever you do, DO NOT close Firefox. Firefox keeps the information on your open tabs in a file. When you close Firefox it updates that file with the windows and tabs that are currently open – and doing that will overwrite the information you are trying to restore. Leave the windows and tabs you have open as they are. Complete the restoration of your previously opened tabs and then close what you don’t need.