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Getting a Firefox extension’s directory from within the extension

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When creating a Firefox extension recently, I was surprised how much pain is involved simply in finding out, from within an extension, where that extension’s files are installed.

You have to create a ‘component’, which entails a fair chunk of unintelligible boilerplate code (well, unintelligible unless you’re much better acquainted with Firefox’s innards than developing a basic extension generally requires you to be).

Plenty of places will tell you roughly how to do it. But, after some experimentation, let me show you exactly how.

  • Copy this JavaScript file into your extension’s /components directory (if that directory doesn’t exist yet, create it).
  • Add these lines to your chrome.manifest:
component {723079F5-F880-40BB-8283-8266DEA93960} components/getExtDir.js
contract;1 {723079F5-F880-40BB-8283-8266DEA93960}
  • Use it from elsewhere in your extension like so:
var own_path = Components.classes[";1"]

Update, August 2011 — Firefox 3 made this a bit less hairy, and Firefox 4 introduced a small but incompatible change. The updated code above works in Firefox 3 – 6, and most likely beyond. However, to make knowing your own directory useful (e.g. in order to locate and run an AppleScript there) you may have also to set <em:unpack>true</em:unpack> in install.rdf. This activates the old extension unpacking behaviour, which is deprecated because it increases load time.

Written by George

May 7th, 2009 at 1:23 pm

Posted in JavaScript