Get "FEBE" add-on.
It can 're-create' nearly every setting and appearance just as you have it on your current setup.
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"; b.u. '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.
Copy that desktop folder to a USB, then migrate to the new unit.
Install Firefox + FEBE on that, then grab that folder and restore files to the new unit.