how to disable Firefox's app tab glow?

Solution 1:

Put one of these snippets in your userChrome.css file, depending on your Firefox version:

Firefox 69

Toggle toolkit.legacyUserProfileCustomizations.stylesheets to true in about:config as well as userChrome.css modification.

Firefox 29 (Australis)

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

Pre-29

#tabbrowser-tabs > .tabbrowser-tab > .tab-stack > .tab-background * {
    background: none !important;
}

Discovered by looking for [titlechanged] selectors in browser.css (chrome://browser/skin/browser.css).

1. https://www.userchrome.org/how-create-userchrome-css.html
2. https://bugzilla.mozilla.org/show_bug.cgi?id=1541233