Browser-Components will run in your own Process (hence you can influence the DOM per Inprocess-calls to these Componets/Libraries).
But you ruled them out.

Seems what you want, is to interact with the DOM in a *foreign* Process (Firefox.exe) -
and the only way to do that (without true hacking) is, to "get your own component into the Firefox-Process".

And yes, there is an official way to do that, you know...
https://blog.mozilla.org/addons/2014...fox-extension/

Olaf