Bottom Line: One can write a Firefox extension in (mostly) Python via Pyodide.

Disclaimer: I’m not a huge fan of JavaScript, and I don’t use it much, so I am likely not following best practices. I’ve also never written a Firefox extension, so the below is pretty bare-bones, but hopefully enough to get you off the ground.

With the recent amazing advancements in Python and wasm, brought to us in large part by way of Pyodide (repo) and PyScript (repo), I thought it would be interesting to try to build a Firefox extension in Python.

I found a very helpful Medium article and corresponding GitHub repo for building a Chrome extension in Python, which provided some examples and a framework. I wanted to do things a little differently (for no good reason) – specifically, I didn’t want to rely on an html-based pop-up page, which that project uses to load all the JavaScript files.

I struggled to get PyScript to work in the way I wanted, but I was eventually able to get Pyodide to help me create an extension that contains its own Python wasm runtime (and therefore doesn’t need to load it from their web-hosted version and should be a little snappier to load in some cases).

To try out the toy extension:

  1. clone the example repo, which is at
  2. inspect (for safety) (MacOS / probably Linux) or setup.ps1 (Windows) and afterwards run them; this will download the necessary files from Pyodide so you can embed them in your extension.
    • You can also consider changing the script to download the debug version during development
  3. open Firefox to about:debugging
  4. click the link for This Firefox
  5. click Load Temporary Add-on...
  6. Select the manifest.json from the cloned repo

In short, I found that you can import pyodide.js in your manifest.json using a local path. That defines a function loadPyodide, which can accept an object with an indexURL argument. manifest.json then loads a local JavaScript file, hello.js, which calls loadPyodide with indexURL set to a local path to the rest of the necessary files.

From here, loading and running some Python is a little janky, but seems to work – I just read the contents of and pass it (as a string) to pyodide.runPython. One reason I wanted to structure things this way is it allows me to use my usual Python workflow to write / edit / lint / format the Python code.

In, I demonstrate very basic functionality for both a content_script extension, which can modify the content one sees, as well as a background extension, which has access to inspect, open, and close tabs (among many other things). To demonstrate the content_script functionality, the extension sets a red border around the currently open webpage. In manifest.json, I restrict the extension to only run this content script on, so if you open a page to my site you should see a red border.

For the background script functionality, I print out a list of currently open tabs into the devtools console; to view this, click the Inspect button in Firefox’s about:debugging tab, then go to the Console tab. I also print out the current webpage’s URL, and open a new page to this blog post (which should get a red border).

This was a fun project, if a little frustrating to sort out (given my unfamiliarity with JavaScript). If you have any recommendations or other example projects, I’d love to hear about it in the comments below!