The controversy at the time was that it didn't need to be embedded in the browser and it was a proprietary blob.

It literally took years to get to a point where the bundled parts were released as open source (see https://bugzilla.mozilla.org/show_bug.cgi?id=1343006 ). Obviously there are legal concerns etc, but many of the critics would have been much happier if the extension were released as open source first rather than adding proprietary code to the browser