Verify proxy config switching end-to-end in Firefox

[infralayers]

Goal

Confirm the example extension loads and PAC-based config switching works in Firefox. Proxy auth is a known Chrome-only gap right now (see issue for Firefox auth parity) — this issue is scoped to everything except auth.

Steps

1. npm install && npm run build --workspace=packages/core && npm run build --workspace=packages/example-extension
2. Follow packages/example-extension/README.md "Load in Firefox" section
3. Confirm the extension loads in about:debugging with no errors
4. Submit the popup form and confirm the PAC script config switch takes effect (proxy auth itself is expected not to work yet — that's tracked separately)

Definition of done

Comment on this issue confirming what worked and what didn't, so we know exactly where the Firefox auth-parity work (separate issue) needs to start from.

Suggested owner: the other of you — split Chrome/Firefox verification between the two of you.