Save My Tabs is a great Firefox extension, because Firefox is great, but crashes a lot (at least mine, with hundreds of tabs).
Unfortunately, Save My Tabs doesn't have any restoration function, so you can copy/paste all URLs yourself, or try this script.
This simple Shell script for Mac OS X will launch Firefox with all URLs present in the backup file you pass as parameter:
./restore-savemytabs.sh opentabs--20160913-1900.txt
Tabs groups (I use Simplified Tab Groups since Panorama deprecation), if you use some, are unfortunately not restored.