Review Mozilla 1990309: Promote ESLint rule mozilla/no-browser-refs-in-toolkit to an error, except for existing files where it fails

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Bugzilla: https://bugzilla.mozilla.org/show_bug.cgi?id=1990309

Mozilla has this mozilla/no-browser-refs-in-toolkit linter rule which this ticket turns from warning to error.

We have some occurances of this as an error.

I turned it back to a warning for now, but we should enable and fix our occurances if we realize it makes sense for us.

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