add firefox tab size change

code-style.md

@@ -8,3 +8,17 @@
 * Use `foo` rather than `this.foo` unless there's also a local `foo`
 * `CONSTANTS_LIKE_THIS`, `variablesLikeThis`
 * `AbcCamelCase` rather than `ABCCamelCase`
+
+# Firefox Tab Size in Gitlab
+
+Create `$HOME/.mozilla/firefox/yrj5n7s5.default/chrome/userContent.css` (if it doesn't exist) and add:
+
+```css
+@-moz-document domain(code.briarproject.org) {
+    .line {
+        -moz-tab-size: 4;
+    }
+}
+```
+
+Restart your browser and enjoy a custom tab size in Gitlab's code and diffs.