We had that ability in Firefox, through XUL. Then it was removed. Tree Style Tab addon doesn't work properly to this day because of this.

We had that ability in Chrome, through Chrome Apps. You could make a browser app, load pages in webviews, with the whole browser frame customizable. Then it was removed.

We had an ability to make a new innovative browser, until Google infested all the standartization committees, and increased complexity of standards on a daily basis for well over a decade. Now they monetize their effort on making Chrome by removing adblockers and enforcing their own ads, knowing full well that even keeping a fork that supports manifest v2 is infeasible for a free open-source project.

There is no way forward with the web we have right now. No innovation will happen anymore.