- Create a local build of MetaMask using your preferred method.
- You can find build instructions in the readme.
- Go to the url
about:debugging#addons. - Click the button
Load Temporary Add-On. - Select the file
metamask-extension/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.