I had the same issues as you and found a nice solution. It involves no addon and no 3rd party.
It's a python script which simply decrypts the passwords in your firefox profile (you have to input the master password if you use one).
On linux (probably the same on mac, don't know on windows), just type python firefox_decrypt.py -t and it will dispaly in the terminal window all your passwords. Select all the lines and copy them in a txt file.
You just need to tweak it a little : delete the comma at the end of each line and single quotes around logins and passwords (a simple search and replace is fine).
And you're ready to import all your passwords in enpass. I did it with 563 lines and it worked like a charm.
Would be great if it could be added as a native feature.
Have a great day !