Advantages of the SDK
Although we can't promise we'll never break a High-Level API, maintaining compatibility across Firefox versions is a top priority for us.
We've designed the APIs to be forward-compatible with the new multiple process architecture (codenamed Electrolysis) planned for Firefox.
We also expect to support both desktop and mobile Firefox using a single edition of the SDK: so you'll be able to write one extension and have it work on both products.
If they're not carefully designed, Firefox add-ons can open the browser to attack by malicious web pages. Although it's possible to write insecure add-ons using the SDK, it's not as easy, and the damage that a compromised add-on can do is usually more limited.
Add-ons built with the SDK can be installed without having to restart Firefox.
Although you can write traditional add-ons that are restartless, you can't use XUL overlays in them, so most traditional add-ons would have to be substantially rewritten anyway.
|User Experience Best Practices||
The UI components available in the SDK are designed to align with the usability guidelines for Firefox, giving your users a better, more consistent experience.
Advantages of XUL-based Add-ons
|User interface flexibility||
XUL overlays offer a great deal of options for building a UI and integrating it into the browser. Using only the SDK's supported APIs you have much more limited options for your UI.
Traditional add-ons have access to a vast amount of Firefox functionality via XPCOM. The SDK's supported APIs expose a relatively small set of this functionality.
Low-level APIs and Third-party Modules
That's not the whole story. If you need more flexibility than the SDK's High-Level APIs provide, you can use its Low-level APIs to load XPCOM objects directly or to manipulate the DOM directly as in a traditional bootstrapped extension.
Alternatively, you can load third-party modules, which extend the SDK's core APIs.
Note that by doing this you lose some of the benefits of programming with the SDK including simplicity, compatibility, and to a lesser extent security.