embedding a CSS file specific to the location using a Firefox add-on

I am currently working on a Firefox extension project and require assistance with inserting elements and CSS into the document.

I have attempted to follow tutorials on injecting a local CSS file into a webpage with a Firefox extension and inserting CSS using a Firefox extension, but have had no success.

I feel like there may be a small detail that I am overlooking, but I'm unable to pinpoint what it is. Any guidance would be greatly appreciated.

Here is my chrome.manifest file:

 content    helloworld content/
overlay chrome://browser/content/browser.xul    chrome://helloworld/content/overlay.xul

locale  helloworld  en-US   locale/en-US/

skin    helloworld  classic/1.0 skin/

Additionally, here is my overlay.js code:

var fileref = gBrowser.contentDocument.createElement("link");
fileref.setAttribute("rel", "stylesheet");
fileref.setAttribute("type", "text/css");
fileref.setAttribute("href", "resource://helloworld/skin/global.css");
gBrowser.contentDocument.getElementsByTagName("head")[0].appendChild(fileref);

I have also attempted the following script in my overlay.js:

var sss = Components.classes["@mozilla.org/content/style-sheet-service;1"]
    .getService(Components.interfaces.nsIStyleSheetService);
var ios = Components.classes["@mozilla.org/network/io-service;1"]
    .getService(Components.interfaces.nsIIOService);
var uri = ios.newURI(url, null, null);
sss.loadAndRegisterSheet(uri, sss.USER_SHEET);

Unfortunately, I am still facing issues.

Can anyone provide insight into what I may be missing? I seem to be at a standstill.


  • Attempts to use the console yielded no results.
  • After pasting the href "chrome://helloworld/skin/global.css", I could view my CSS file in the browser.

Answer №1

It is recommended to adjust the javascript.options.showInConsole setting, then restart and review the Error Console for any potential issues.

Implementing the nsIStyleSheetService snippet is usually the quickest solution to get things working smoothly.

Ensure you are using the correct url in your code. It seems there may be some inconsistencies in the URLs provided, such as chrome.manifest and resource://, which can lead to confusion.

If you suspect the snippet is not functioning properly, double-check your stylesheet for errors. Start with a simple CSS rule like * {color:red !important;} to test if the styles are being applied correctly.

Keep in mind the caution mentioned on MDC regarding registering the same stylesheet multiple times when utilizing nsIStyleSheetService.

Be cautious with nsIStyleSheetService to avoid unintended style changes on other pages. Utilize @-moz-document for targeted styling when needed.

Answer №2

If you're looking for a potential solution, consider implementing a method to listen for load events in all tabs by adjusting your overlay.js script like this:

window.addEventListener('load', function () {
gBrowser.addEventListener('DOMContentLoaded', function (event) {
  if (event.originalTarget.nodeName == '#document' && 
     event.originalTarget.defaultView.location.href == gBrowser.currentURI.spec)
  {
    var document = event.originalTarget;
    // Implement your CSS injection code here
  }
}, false),
true);

Answer №3

Do away with the overlay.js and overlay.xul files, they are unnecessary. Opt for using the chrome.manifest style declaration instead, like this:

style chrome://browser/content/browser.xul resource://helloworld/skin/global.css

No JavaScript required

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