Understanding Firefox extension structure

Question

I'm trying to write a Firefox extension that intercepts a certain HTTP request and return static content without the request making it to the actual server (similar to AdBlock).

I've looked up the tutorials and I've got a basic file layout. I've also worked out that I need to use the nsITraceableChannel API and add an observer to do what I want and I have example code for that.

Problem is, where do I actually put this code? And when is my extension actually loaded and executed? Is it running constantly and asynchronously in the background or is it loaded per page view?

The documentation doesn't seem very clear on this. This extension won't need a GUI so I don't need the layouting XUL files (or do I?). I tried writing some XPCOM (I don't think I did it right though) component, registered it in chrome.manifest but it doesn't seem to run.

Can anyone explain exactly how the Firefox extensions work and where should I put my actual JavaScript code to monitor requests? Or have I got the whole idea of what an extension is wrong? Is there a difference between add-ons, extensions and plugins?

Answer 1

Concerning the difference between add-ons, extensions and plugins you should look at this answer. But in general, you seem to have the correct idea.

The problem is, there are currently three very different types of extensions:

1. Classic extensions (not restartless): these will typically overlay the browser window and run code from this overlay. Since there is one overlay per window, there will be as many code instances as browser windows. However, classic extensions can also register an XPCOM component (via chrome.manifest as of Gecko 2.0). This component will be loaded on first use and stay around for the entire browsing session. You probably want your component to load when the browser starts, for this you should register it in the profile-after-change category and implement nsIObserver.
2. Restartless extensions, also called bootstrapped extensions: these cannot register overlays which makes working with the browser UI somewhat more complicated. Instead they have a bootstrap.js script that will load when the extension is activated, this context will stay around in background until the browser is shut down or the extension is disabled. You can have XPCOM components in restartless extensions as well but you will have to register them manually (via nsIComponentRegistrar.registerFactory() and nsICategoryManager.addCategoryEntry()). You will also have to take care of unregistering the component if the extension is shut down. This is unnecessary if you merely need to add an observer, nsIObserverService will take any object implementing nsIObserver, not only one that has been registered as an XPCOM component. The big downside is: most MDN examples are about classic extensions and don't explain how you would do things in a restartless extension.
3. Extensions based on the Add-on SDK: these are based on a framework that produces restartless extensions. The Add-on SDK has its own API which is very different from what you usually do in Firefox extension - but it is simple, and it mostly takes care of shutting down the extension so that you don't have to do it manually. Extensions here consist of a number of modules, with main.js loading automatically and being able to load additional modules as necessary. Once loaded, each module stays around for as long as the extension is active. They run sandboxed but you can still leave the sandbox and access XPCOM directly. However, you would probably use the internal observer-service module instead.