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In Mozilla Firefox, I want to create userContent.css which overrides the CSS of a site, say this one.

Where should I create the file? Could you please describe it based on Windows 7?

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2 Answers 2

up vote 30 down vote accepted
  1. Open Firefox and press ALT to show the top menu, then click on HelpTroubleshooting InformationOpen Containing Folder
  2. Create a folder named Chrome
  3. Create a css file with the name UserContent.css
  4. Copy the following code to UserContent.css, replacing "example.com" with the website you want to modify and your own custom CSS, and restart Firefox:
@-moz-document domain(example.com) {
    img{opacity: 0.05 !important;}
}
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5  
A small update / modification for step 1; the Troubleshooting Information opens up a link about:support and in my Firefox (v20 nightly build on Ubuntu) it has an entry named "Profile directory" with a button "Open Directory" which opens up the profile directory /home/tripleee/.mozilla/firefox/asdf1234.default/ in Nautilus. –  tripleee Jan 3 '13 at 10:38
    
You can remove @-moz-document domain("...") to apply the style to every pages. –  Nicolas May 30 '13 at 8:26
1  
Why is it supposed to be in the chrome folder? –  Septagram Dec 18 '13 at 8:01
    
@Septagram I think it's just always been that way… as I remember, chrome could also contain userChrome.css, which would style the browser interface ("chrome"). –  sam Sep 27 '14 at 18:05
    
I know the OP asked about Windows 7, but for OS X users the path is ~/Library/Application Support/Firefox/Profiles/<profileid>.default/chrome/ –  rlovtang Dec 1 '14 at 15:54

The add-on ChromEdit Plus will do all this for you.

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