Add Firefox versions for api.Document.execCommand.insertBrOnReturn (#12868)
Add Firefox versions for api.Document.execCommand.insertBrOnReturn (#12868)

This PR adds real values for Firefox and Firefox Android for the `execCommand.insertBrOnReturn` member of the `Document` API, based upon information in a tracking bug.

Tracking Bug: https://bugzil.la/285873
mdn/browser-compat-data,chrisdavidmills/browser-compat-data,chrisdavidmills/browser-compat-data,mdn/browser-compat-data