Firefox (MV3) compatibility

[theonlytruebigmac]

Description

Port the extension to work on Firefox using MV3 (Manifest V3) APIs, handling differences in storage, background scripts, and permissions.

Scope

• Audit Chrome-specific APIs and find Firefox equivalents
• Handle chrome. vs browser. API namespace differences
• Test with Firefox Developer Edition
• Create Firefox-specific manifest if needed (or use WebExtension polyfill)
• Submit to AMO (addons.mozilla.org)

Acceptance Criteria

• Extension loads and runs in Firefox Nightly/Developer Edition
• All core features work: scraping, enrichment, export
• Passes AMO automated validation
• Single codebase works for both browsers (or minimal branching)

Part of #4