Earlier quoted context omitted.
It has to be done by browser vendors, otherwise it will not be adopted.
It's not about browsers or tech, the extensions were universally available with a single click on modern browsers.
1. Locate and open Addons screen (~2-3 clicks in the browser menu).
2. Locate and click "Get more addons" (1 click).
3. Wait for store page to open, search for desired extension. (Around a dozen keypresses for average-length extension name.)
4. Locate and click "Install" (1 click).
5. Confirm modal dialogs for installation and permissions (1-2 clicks).
That's nowhere near one-click and even if it were a one-click operation, 90% of users don't even know they would want to do that click.