First, open the menu at the top right of Firefox and click "Preferences". This will open up a new tab with all of Firefox's preferences available.
On the left side of the Preferences screen, click on "Privacy & Security".
Scroll to the bottom and click the button that says "View Certificates...". This will open a modal.
In the modal, switch to the "Authorities" tab. Then click the "Import..." button.
Select the file at $DEVCA_HOME/ca/cert.pem and import it. If you don't know this location, you can run devca path-to ca --cert to find it.
That's it! Firefox should now trust any certificates generated by devca. Note that if you run the regen command, you will have to follow these steps again with the new CA certificate.