Unfortunately the speaker icon is part of Firefox' mute functionality and appears automatically. As far as I know, it's not yet possible to disable it using WebExtensions.

You could hide it yourself though by adding something like this

@namespace url(http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul);
.tabbrowser-tab :-moz-any(.tab-icon-sound,.tab-icon-overlay[muted]) {
display: none;
}

to your userChrome.css (https://www.userchrome.org).