feat[react-devtools]: support Manifest v3 for Firefox extension (#30824)

Firefox [finally supports
`ExecutionWorld.MAIN`](https://bugzilla.mozilla.org/show_bug.cgi?id=1736575)
in content scripts, which means we can migrate the browser extension to
Manifest V3.

This PR also removes a bunch of no longer required explicit branching
for Firefox case, when we are using Manifest V3-only APIs.

We are also removing XMLHttpRequest injection, which is no longer needed
and restricted in Manifest V3. The new standardized approach (same as in
Chromium) doesn't violate CSP rules, which means that extension can
finally be used for apps running in production mode.