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You can create patches that change the interface and/or propose UI changes. They will go through an additional ui-review though. Sometimes the result will be that writing an add-on would be a better fit for a feature or UI change.

Also, if your proposal is around reverting something to a pre-29 state, it has probably already been discussed a lot.

Everything is discussed via meetings and mailing lists. I guess you can not make significant changes to the UI, but you can contribute by writing an extension.

Given that people have written add-ons to do just that, there's no need to even make patches for it.

The "Classic Theme Restorer" add-on works pretty well and has a lot of preference to tweak the UI to your liking:

https://addons.mozilla.org/en-US/firefox/addon/classicthemer...