The mute icon (i.e. speaker) is useful but its location on the tab is causing some people to accidentally mute tabs. Currently there is some free clickable space on both sides of the icon, but it would make more sense to have a wider continuous free area on the tab. To achieve this IMO ideally the icon should be at the right end of the tab to the left of x. This is where the icon was always located in previous versions of Firefox about 4 years ago.
The icon should also have smaller borders but that's a different issue.