Hi. In this image there are 3 screenshots of the same browser tab in different states. The tab is the first one in the list.
In example [1] no audio is playing.
In example [2] audio is playing and because the audio indicator pops up the tab widens, pushing the "close tab" button part of the tab behind the "scroll forwards" arrow instead of widening the tab away from it. This behavior is, to me a least, very annoying. Example [3] is of course how it's intended too look.
Example [4] is a suggestion for how I think tabs should look when audio is being played. No need for annoying size changes. To mute the tab you can click on the top green line to make it red. Maybe the line should be one or two pixels wider, but it's just an example.
Link to reddit post: https://www.reddit.com/r/firefox/comments/1kp5nnb/opinion_the_new_dynamic_tab_size_audio_vs_no/