Instruction on how to create custom firefox css rules

firefox-custom-css.txt

Current goal is to replace Helvetica font, need to put this into userContent.css:
```css
@font-face {
  font-family: "Helvetica";
  src: local("sans-serif");
}
```

Step 1: Locate and Open Profile Directory

The user stylesheet will be added to your each user profile, not the browser code itself; this makes sense since each profile may want a different UI modifications. In the address bar, visit about:support and click Profile Folder's "Show in Finder" (or likewise) button:

about:support Firefox

The directory will be opened on your system in Finder (Mac) or Explorer (Windows).
Step 2: Create chrome Directory

Create a chrome directory within the designated profile directory which will host the necessary userContent.css file:

Firefox profile chrome

Having the userContent.css file in this directory will allow the user stylesheet to not be lost during Firefox upgrades.
Step 3: Create userContent.css

Firefox adds the userContent.css file on every page, if it exists, so add userContent.css within the chrome directory. This file should host all modifications to any page you visit, so don't create overly broad CSS selectors which will apply to every website.
Firefox stylesheet

/* an overly aggressive style just to prove it works */
body {
  background: pink !important;
}

Firefox stylesheet
Step 4: Update about:config

In the address bar, open about:config and set the toolkit.legacyUserProfileCustomizations.stylesheets setting to true.
Step 5: Restart Firefox

User styles aren't applied to sites until you restart Firefox. Once Firefox has been restarted, the styles in your userContent.css will be applied to the page.