Adds Firefox as browser to `dev-infra/browsers` with RBE
compatibility. The default Firefox browser is not compatible similar to
the default Chromium version exposed by `rules_webtesting`.
The Angular Components repository will use this browser target as
it enables RBE support. Also it gives us more flexibility about
the Firefox version we test against. The version provided by
`rules_webtesting` is very old and most likely not frequently
updated (based on past experience).
PR Close#38029
Paul Gschwendtner