It became unused in bug 1917913

Thanks Gregory for the clean up.
However, my report, as good first bugs with details, were probably more likely to be fixed.
Are you ok if i update these bugs?

Yes.

Repurposing this as a good first bug, per comment 3/comment 4 — thanks Gregory.

The preference lightweightThemes.getMoreURL is defined in browser/app/profile/firefox.js but is never read anywhere in mozilla-central. It became unused in bug 1917913. Please remove its definition line.

Link to the code:
https://searchfox.org/mozilla-central/source/browser/app/profile/firefox.js#239

To verify the fix:

• Confirm the pref has no remaining users by searching for its name on
  https://searchfox.org/mozilla-central/search?q=lightweightThemes.getMoreURL
  (only the definition should match).
• Build the tree to make sure nothing breaks:
  ./mach build

Tutorial to contribute:
https://firefox-source-docs.mozilla.org/contributing/contribution_quickref.html
https://firefox-source-docs.mozilla.org/contributing/stack_quickref.html

Please don't ask for the bug to be assigned. It will be automatically assigned to the first patch.

I confirmed the pref isn't used anywhere by doing a code search.
The build passed.

https://hg.mozilla.org/mozilla-central/rev/9ad1b0b9bfcc