You can use Firefox nightly and create a webextension using their internal APIs. Gives you access to injecting non-synthetic or faked events (they look indistinguishable since they are emitted in the same code path that the real input device uses) and scraping the DOM with the added advantage of not exposing the automation flags that Firefox has when you use Puppeteer and the like in headless, automated mode.

edit: Firefox Nightly because it gives you much deeper access to Firefox internals.