Add Firefox-targeted validation and release-readiness checks

[bdtran2002]

Summary

Add a repeatable Firefox validation pass for the extension runtime.

Why

The product is Firefox-first, but there is still no established validation workflow for the extension runtime, popup flow, permissions, or manifest behavior.

Scope

• validate temporary add-on loading flow in Firefox
• verify popup, options, storage persistence, and background search flow end to end
• review manifest and permission assumptions against current runtime behavior
• document the manual validation checklist in the repo

Acceptance criteria

• a documented Firefox validation checklist exists
• the main capture/search flow is verified against the current manifest/runtime setup
• repo docs reflect the expected validation process for future changes