Firefox 115+ still read the userChrome.css file, but the built-in style rules change from time to time, so you need to update your rules in userChrome.css.

If NONE of your userChrome.css rules work at all, two common reasons would be:

(1) The file is not in the expected location inside your currently active profile folder. You can confirm your current profile folder using the Troubleshooting Information (about:support) page. See:

Use the Troubleshooting Information page to help fix Firefox issues

(2) The toolkit.legacyUserProfileCustomizations.stylesheets preference is not set to true. This needs to be set to true or Firefox won't look for userChrome.css at startup. Configuration Editor for Firefox

If you can rule out those issues, then it's probably the specific rules that need updating. A possibly better place for advice on that is:

https://www.reddit.com/r/FirefoxCSS/

Localising the file: i asked GPT, who said to check about:profiles, which points to /home/dhp/.mozilla/firefox/MySecret.default . In this folder, I edit chrome/userChrome.css . But your about:support gives the same result.

in about:config, toolkit.legacyUserProfileCustomizations.stylesheets was false. Switching it, sending this post, and restarting FF ... CU in 10s ...

toolkit.legacyUserProfileCustomizations.stylesheets did the job, thanks.

Here is a sample that orks for me, on short term. Will customise more later.

/* https://www.reddit.com/r/FirefoxCSS/comments/x42ojc/tabs_border_outlining_now_working_in_firefox/ https://support.mozilla.org/en-US/questions/1270436

• /

.tabbrowser-tab .tab-background { border: 1px solid red !important;

border-radius: 3px 3px 0px 0px !important;
border-image: none !important;
border-left: 0px solid !important;
border-right: 0px solid !important;
border-top: 1px solid !important;
background-color: #BDBDBD !important;

} /* https://support.mozilla.org/en-US/questions/1282095 */ /*.tab-line[selected="true"] {

     background-color: red !important;
  }*/
 .tab-background[selected="true"] {
     background-color: #E0E0E0 !important;
     background-image: none !important;

border: 1px solid red !important;

   }

Note that you may have to leave out ="true" in [selected="true"] and use [selected] to avoid issues with newer Firefox versions.