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Also, about:config is disabled on mobile unless you're running a nightly build.

Mull (https://github.com/divested-mobile/mull-fenix) is a great fork with privacy improvements, and about:config is accessible.

Does it have all the usual codecs, or are some of them considered too proprietary to give users the option of using them?

You can access about:config and use add-on collections on Firefox Beta too now.