Hello, kosakian

Ahh, so that add-on is the culprit  !

I don't blame you for not wanting to try that CSS - should you want to try (maybe later) :

https://www.howtogeek.com/334716/how-to-customize-firefoxs-user-interface-with-userchrome.css/

In the mean time :

You could try to contact the developer of the add-on : see under 'More information' => Support Email

https://addons.mozilla.org/en-US/firefox/addon/auto-mute/

But you say that you uninstalled it - problem solved then  ?

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 */

/* Reorder tab icons */
.tab-content .tab-label-container {-moz-box-ordinal-group: 2; margin-inline-start: 3px !important}
.tab-content .tab-icon-sound {-moz-box-ordinal-group: 1}
.tab-content .tab-close-button close-icon {-moz-box-ordinal-group: 3}

.tab-throbber:not([pinned]),
.tab-sharing-icon-overlay:not([pinned]),
.tab-icon-pending:not([pinned]),
.tab-icon-image:not([pinned]) {
  margin-inline-end: 3px !important;
}

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