"FEBE" add-on will copy almost all of the Firefox environment from the old, and move it to the new system, making the new Ffox just like the old one.
Look in https://addons.mozilla.org/en-US/firefox/addon/2109/
On your desktop, create a folder ("FEBE" or your choice).
Then, in FF, > Tools> FEBE> FEBE options> Options tab: for a complete BU of all items (for re-creating Firefox on another system), tick "Both"; bu 'extensions' & 'themes'; and Additional items, drop menu, "Select all" @ the bottom of window.
You also need to set the 'destination' while here:
Top tabs, "Directory"> Browse> (locate that desktop folder you made). The filepath should now be in the top slot.
To back up: In FF> Tools> FEBE> Perform backup.
This is assuming you can get the old OS up (somehow).
Next time use FEBE regularly, saved to external media, so you don't have meltdowns.