The road to out-of-process WebExtensions

Rob Wu <rob@robwu.nl>

Who is Rob Wu?

• MSc student Information Security Technology @ TU/e (Netherlands)
• Chromium contributor, extension developer
• Reviewer at addons.mozilla.org
• Firefox intern

WebExtensions

• Cross-browser API to develop browser add-ons
• Successor to XUL overlays, bootstrapped extensions and Add-on SDK
• Shields the volatile internals of Firefox from add-ons
• Compatible with multi-process Firefox (e10s)
• Well-documented

Out-of-process WebExtensions

• Goal: run add-ons in a separate process
• Must stay compatible with single-process Firefox
• Getting started:
  • Check correctness of existing implementation
  • Unique IDs must really be unique (1287245, 1287626, 1288279)
  • Script contexts must close upon page unload (1288276)
  • Namespace isolation for code in different "processes"

Registering API implementations

• Script types: content scripts & extension pages
• Separate JS sandbox for each "process" (main, content, addon)
• Declare APIs with specific script and process types (1287010)

  extensions.registerSchemaAPI("runtime", "content_child", (context) => {
    return {
      runtime: {
        get id() { return context.extension.id; },
        sendMessage(message, callback) { /* ... */ },
      },
    };
  });
      
    
  

Thanks to

• Bill McCloskey
• Andy McKay
• WebExtensions team
• Amy Tsay & ACE team
• Fellow interns (Enes, Emilio, Nima, ...)
• University team
• Mozilla and Mozillians