David Kitchen

This is a rather good file, and you can find it on your Windows system by browsing to:
%appdata%\Mozilla\Firefox\Profiles\profilename\chrome

By default there is an example file name userContent-example.css that contains commented out examples:
/*
* example: turn off "blink" element blinking
*
* blink { text-decoration: none ! important; }
*
*/

Effectively this is a global Cascading Style Sheet file that will be applied to every site that you visit.

Initially it’s hard to see the point in such a thing, but after a while it becomes apparent that this is a rather sneaky way to customise the entire www a little to your desires.

I have a pet hate for PDF files, the length of time it takes for Acrobat Reader to fire up, and the deceptive way that a lot of links point to them. So I like it when sites indicate that a link points to a PDF file. Too few sites do this though. That’s OK, with the userContent.css file we can create a stylesheet entry that will detect PDF files and add some text for us so that we can spot PDF links before we click on them:
/*
* Warns about PDF links
*/
a[href$=".pdf"]:after {
font-size: smaller;
content: ” [pdf]“;
}

Now whenever I see a link to a PDF file, that link is followed by the text: [pdf]

Lovely.

Here are some of the other things I’ve put in my userContent.css file:
/*
* Warns about PDF links
*/
a[href$=".pdf"]:after {
font-size: smaller;
content: ” [pdf]“;
}
/*
* Warns about new window links
*/
:link[target="_blank"]:after, :visited[target="_blank"]:after, :link[target="_new"]:after, :visited[target="_new"]:after {
font-size: smaller;
content: ” [new]“;
}
/*
* Warns about JavaScript links
*/
a[href^="javascript:"]:after {
font-size: smaller;
content: ” [js]“;
}
/*
* Hide MS Ads on TheRegister.co.uk
*/
div[id="ad"] {
display: none ! important;
}
/*
* Fixes bug in Firefox
*/
col {
display: none ! important;
}

To set up a userContent.css file, just add a text file named that to the chrome directory in your settings:
%appdata%\Mozilla\Firefox\Profiles\profilename\chrome\userContent.css

Within that you should place your CSS rules.