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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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up vote 40 down vote accepted
  1. Open Firefox and press ALT to show the top menu, then click on HelpTroubleshooting Information
  2. Click the Show Folder button beside the Profile Folder entry
  3. Create a folder named Chrome in the directory that opens
  4. In the chrome folder create a css file with the name userContent.css
  5. Copy the following code to userContent.css, replacing "" with the website you want to modify and your own custom CSS, and restart Firefox:
@-moz-document domain( {
    img{ opacity: 0.05 !important; }
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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
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

Alternatively, you can use the Stylish extension on Firefox or Chrome.

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I thought I'd got ChromEdit from the official list, but OK. – OrangeDog Jul 2 '15 at 19:57

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