I'm happy to mentor work towards this bug. Please don't ask to be assigned to it, though you can say you're working on it. You'll be automatically assigned when you upload the first patches.

Here's what to do:

• Make sure you have a local build of Firefox up and running. Use these instructions if you haven't already.
  • If you need help setting up, ask in #introduction on Matrix
• Using the ESMification walkthrough as a guide:
  1. Run ./mach esmify --convert path/to/files
  2. Examine the output for any errors from ESLint and fix them.
  3. Next convert the imports - ./mach esmify --import . --prefix path/to/files
  4. Again, check for any errors and fix them.
  5. Run a build to check it passes.
  6. Also run ./mach eslint --fix path/to/files and check there are no raised failures
  7. Please also update the documentation, specifically this line to change .jsm to .sys.mjs
  8. Commit the changes with a commit messages such as "Bug nnnnnn - Convert path/to/files to ES modules. r?Standard8"
• Use moz-phab submit to submit the patches for review.

https://hg.mozilla.org/mozilla-central/rev/7d0fcc4ba0b1