Sadly, modern browsers don't allow add-ons to significantly alter the UI, so you'd never see an extension like this today.

You can do some neat stuff with userchrome in Firefox (not as an add-on) but it'll break after basically every update. There's no infrastructure in place to update the userchrome in sync with the browser, and Mozilla appears to be hostile towards the whole feature.

There are decent reasons for all of this, but it means the only way to experiment is to basically make your own app, as Min did.