Go to the url about:debugging.
Click the button Load Temporary Add-On.
Select the file dist/firefox/manifest.json.
You can optionally enable debugging, and click Debug, for a console window that logs all of Metamask's processes to a single console.
If you have problems debugging, try connecting to the IRC channel #webextensions on irc.mozilla.org.
For longer questions, use the StackOverflow tag firefox-addons.