Verify and improve Firefox support

[lvchenjia]

Description

The app currently lists Firefox as a known browser but doesn't have dedicated AppleScript support. It falls back to the generic open location command. We should verify it works correctly and add tab-specific handling if possible.

Tasks

• Install Firefox and test current routing behavior
• Verify the open location AppleScript command works in Firefox
• Optionally add Firefox-specific tab behavior (similar to the Safari tab implementation)
• Update README.md with any Firefox-specific notes

Resources

• Router code in Sources/BrowserProxy/Router.swift
• Look at the Safari tab implementation for reference

Testing

• Set default browser to Firefox, click a link, verify Firefox opens the URL
• Test with Firefox already running vs not running