This is the CSS code that adds the 'dot':

• https://dxr.mozilla.org/mozilla-release/source/browser/themes/shared/tabs.inc.css#601

You can add code to the userChrome.css file below the default @namespace line.

@namespace url("http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul"); /* only needed once */

/* Suppress blue dot on pinned tab */
.tabbrowser-tab > .tab-stack >
 .tab-content[pinned][titlechanged]:not([selected="true"]){
  background-image: none !important;
}

It is not that difficult to create userChrome.css if you have never used it.

The first step is to open the "Help -> Troubleshooting Information" page and find the button to access the profile folder.

• https://support.mozilla.org/en-US/kb/use-troubleshooting-information-page-fix-firefox

You can find this button under the "Application Basics" section as "Profile Folder -> Open Folder". If you click this button then you open the profile folder in the Windows File Explorer. You need to create a folder with the name chrome in this folder (name is all lowercase). In the chrome folder you need to create a plain text file with the name userChrome.css (name is case sensitive). In this userChrome.css text file you paste the text posted. On Mac you can use the TextEdit utility to create the userChrome.css file as a plain text file.

In Windows saving the file is usually the only time things get more complicated because Windows can silently add a .txt file extension and you end up with a file named userChrome.css.txt. To avoid this you need to make sure to select "All files" in the dialog to save the file in the text editor using "Save File as".

You need to close (Quit/Exit) and restart Firefox when you create or modify the userChrome.css file.

See also:

• https://www.userchrome.org/what-is-userchrome-css.html
• https://www.userchrome.org/how-create-userchrome-css.html

In Firefox 69 and later you need to set this pref to true on the about:config page to enable userChrome.css and userContent.css in the chrome folder.

• toolkit.legacyUserProfileCustomizations.stylesheets = true

See:

• https://www.userchrome.org/firefox-changes-userchrome-css.html

I meant for an individual tab. Thanks anyway.

You can add a label specifier if you want to target a specific pinned tab.

.tabbrowser-tab[label^="Gmail"] > .tab-stack >
 .tab-content[pinned][titlechanged]:not([selected="true"]){
  background-image: none !important;
}

Is there another way to target a tab, for example by number (placement)? Spotify's heading keeps changing.

[label^="Gmail"] means any label that starts with Gmail. There are more attribute selector possible.

• https://developer.mozilla.org/Web/CSS/Attribute_selectors

Is there a specific pattern for the Spotify labels like unique words ?

I've tried this, but it doesn't work. I've made sure that the url doesn't change.

.tabbrowser-tab[href="https://open.spotify.com/"] > .tab-stack >

.tab-content[pinned][titlechanged]:not([selected="true"]){
 background-image: none !important;

}

I've tried it with href^, too, but to no avail.

href is not a possible attribute that is available for a tab. You can only use the label or part of the label as shown when you hover the tab.

You can use a container and open Spotify link(s) in this specific container as specified by the usercontextid attribute for that container.

.tabbrowser-tab[usercontextid="6"]

See containers.json in the profile folder for more detail about the usercontextid.

• https://support.mozilla.org/en-US/kb/containers

I've ended up using contains the middot as an identifier. It's good enough, but not perfect, because other tabs use it, too. Thank you very much for your help.