Sunday, September 18, 2011

Firefox Browser Extension

Our team has recently built browser extensions for Firefox, Chrome and Safari for I was the author for the Chrome and Safari extensions but I'll try to talk intelligently about Firefox as well.

Building a Firefox extension requires quite a few files and directories. Firefox has an example listing from on their developer site. Here is a directory listing from our plug-in:

 Firefox also makes fairly heavy use of XML configuration files such as the install.rdf file and the XUL GUI files. An interesting exception is the manifest file which is a space delimited file used to register your content with Chrome. Chrome in this case is what Firefox calls the user elements outside the traditional website window which includes toolbars and menu bars. Our looks like this:

Firefox forces you to create your "Chrome" with XUL definitions. XUL is a XML language that allows you to describe your GUI. We ended up creating something like this:

This file is also where you add the link to the Javascript code (narfHelper.js in our case) which will handle all the GUI interactions. You'll notice that we're creating a simple popup menu here which calls functions defined in our Javascript. That is where all the magic occurs.

In the Javascript you have to do things the Firefox way. One such example is displaying popups to the user:

var prompts = Cc[";1"].getService(Ci.nsIPromptService);
prompts.alert(window, "", "Please login and try again");

I actually have no idea where my partner got this code but it looks anything but obvious to me.

Firefox also has support for localizations and various 'skins' which you can achieve through the use of CSS files. It is a powerful system but has a fairly steep learning curve with a lot of additional steps. Thus I think it's the hardest browser of them all to develop extensions for which is ironic considering it probably has the most. However, it was also the first to offer this capability and perhaps the other browsers have learned easier ways to accomplish the same thing. We'll discuss more in a future post about Chrome and Safari.