Advantages of the SDK
| Simplicity |
The SDK provides high-level JavaScript APIs to simplify many common tasks in add-on development, and tool support which greatly simplifies the process of developing, testing, and packaging an add-on. |
| Compatibility |
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. |
| Security |
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. |
| Restartlessness |
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. |
| Mobile Support |
Starting in SDK 1.5, we've added experimental support for developing add-ons on the new native version of Firefox Mobile. See the tutorial on mobile development. |
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. |
| XPCOM |
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.