Firefox-based stealth backend (interest check)

[feder-cr]

hey, before building it i wanted to check if there'd be interest in a Firefox-based stealth backend, parallel to the stealth-chromium option being proposed in PR #5340.

the Firefox build itself already exists and works: feder-cr/invisible_firefox (MPL-2, same license as Firefox upstream, patches applied at the C++ level so there are no JS shims to detect). the Python wrapper is feder-cr/invisible_playwright. the missing piece is the small browser_factory hook plus an env example, which i'd build if there's appetite.

Firefox as a second engine option is useful for sites where Chromium UA gets flagged by default. would be added as BROWSER_TYPE=stealth-firefox, optional dependency, only imported when selected. issues against the backend would route to feder-cr/invisible_playwright rather than here.

also opened a draft PR #6191 with a stub docs page so the proposal has somewhere concrete to land.