What does a common browser add-on that you actually feel you need, do that
you can't find a browser to do if you limited that browser to one type of
web site?

(Treating add-ons, extensions, and plugins the same...)

With browser extensions in the news, I point out that I don't use them
(except where they are incorporated, natively, into the browser use model,
such as is the case with the Tor Browser Bundle).

Instead, I simply set up any one browser (of which there are more than a
dozen to choose from), to work perfectly with one site or type of sites or
set of sites (my choice, depending on the type of sites).

In addition, of course, I have a common MVP Hosts file (or equivalent) ad
blocker on all my machines.

I posit that this use model goes a long way, (perhaps not fully?), toward
eliminating the need for browser extensions, where this post is an attempt
to clarify if that multi-browser use model is viable as a general-use
model.

Hence, I ask the thought-provoking question of...

What does a common browser add-on that you actually feel you need, do that
you can't find a browser to do if you limited that browser to one type of
web site?