Category Archives: Firefox

Using Web Workers in Firefox Extensions

Web Workers allow you to run code in the background in browsers such as Firefox. This is how to build one into a Firefox Extension, which is slightly different than from just creating one as normal on a page. The documentation for doing this is basically non-existent, so hopefully you’ll find this useful.

Please make sure you have a development environment set up similar to the one described here in my previous post.

How do workers work?

  • Workers in /data/ are not directly connected to scripts in /lib/
  • However, they can communicate by sending messages to each other
  • These messages are text only, so could contain serialized JSON, but nothing else
  • You’ll notice below that we are basically just slinging messages between two scripts

The code for the worker

Navigate to the /data/ directory and create a file called hello_world.js

Now paste the following in there (new users of vim, press i  to start typing and Esc  followed by :wq  to save):

This says that whenever the worker receives a message from the client, then send a message back with the word “Hello” prepended.

One note here: In workers, you can’t use the useful function  console.log("message") , instead use  dump("message")

Let’s call the worker from the main code

Let’s navigate back to the /lib/  folder and edit the main.js  file, which is the first thing that runs in the extension.

Paste in the following code:

And run  cfx run . You’ll notice a messy error:

Aha! The key line here is:  ReferenceError: Worker is not defined . This is because Firefox Extensions use something called a ChromeWorker instead. We need to import this in main.js by pasting this at the top:

and changing the line that references the hello_world.js file to call a ChromeWorker instead:

Ok let’s try running it again! Try  cfx run . Wtf another error?!

The key line here is:  Malformed script URI: hello_world.js . This cryptic error is because firefox can’t yet access anything in the /data/  folder. We have to use another part of the SDK to enable access to it.

Open main.js  and put this at the top:

Now we can use the function . When you put a filename as the first argument, it will return a string like  resource://jid1-zmowxggdley0aa-at-jetpack/test/data/whatever_file.js which properly refers to it in the case of extensions. Modify the worker import line as follows:

Now let’s run the extension again using cfx run :

Yay it works! The Worker returned the message “Hello Matthew“.


  • What does this {notation}  mean?

It is shorthand for:

Basically this means that  require("chrome") returns an Object, and we just need the value that is referenced by the key “ChromeWorker”. This is a very succinct way of extracting things from JavaScript Objects that will come in handy in the future.

  • Why is Worker now called ChromeWorker? Are we doing something with Google Chrome?

This is a naming coincidence and nothing to do with Chrome as in the browser. Chrome in this case refers to Firefox Addon internals.