If you are one of those who open more than 100+ tab on Firefox (like me) so a session crash after an update will be one of the worst things ever you could face when you just wake up and open your computer!
In short, This is how to restore Firefox 44 and above crashed session, It may work with older versions as well.
The method of Firefox restore session has changed since 2014 I think! So this method will work on version of 2014 and above.
1- Open Firefox “Open Menu” from the top right.
2- Click on “Open Help Menu”.
3- Choose “Troubleshooting Information”.
4- From “Profile” section, click “Open Folder”.
5- Copy the folder “sessionstore-backups” to a safe place.
In this folder you will find the saved sessions after any crash or upgrade.
6- Exit Firefox by opening the top right menu and click on “Exit Firefox”, then wait a few seconds till it 100% closed.
7- Copy the file which start by “upgrade.js-” and paste it in the root of “Profile Folder”.
8- You can also copy the file named “recovery.bak” depending on if you need to restore last crashed or last upgrade session.
9- Rename the copied file to “sessionstore.js”.
10- Open Firefox again and have fun!