First you need to find your Firefox profile.This works for me. To edit your your userContent.css file: That file is located in the chrome folder of your Firefox profile. Provided you include the ‘ !important‘ directive, the CSS in this file will override the CSS applied by the designer of the web page. Instead, when using Firefox, you can put your custom CSS into the userContent.css file on your own computer.

But then the changes you make are temporary – they last only for a single session. You could add the CSS (see code box below) to your browser using Firefox’s Web Developer, or Greasemonkey, or another web developer tool. They will not affect other users of the site. You can override as few or as many of the CSS styles as you like. Here’s how to override a web page’s CSS styles in your browser. How do you override CSS stylesheets in Firefox, why did we need to do this, and what other ways are available for overriding CSS? How to do it in Firefox So I found out how you can override a web page’s CSS for your browser only. Other readers should see the pages as delivered by the wiki. But we wanted the styling to take effect for our authors only.

Recently we needed to add some custom styling to pages on our documentation wiki.

Firefox css style examples