hackaround that works on the website side: If I add "flex:none" to the mute icon, then I get expected results. It looks like the "collapsed"/"expanded" states of this button just have different widths, and the components have to shrink (using flex layout) in the collapsed state. And in Firefox release, the <input> element does all of the shrinking, whereas in Nightly, the <input> and the <svg> both cooperatively shrink. Hence, you can work around this by explicitly preventing the <svg> element from shrinking (with flex:none whose main effect in this case is to set flex-shrink:0)
Bug 1512066 Comment 6 Edit History
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hackaround that works on the website side: If I add "flex:none" to the <svg> element for the mute icon, then I get expected results. It looks like the "collapsed"/"expanded" states of this button just have different widths, and the components have to shrink (using flex layout) in the collapsed state. And in Firefox release, the <input> element does all of the shrinking, whereas in Nightly, the <input> and the <svg> both cooperatively shrink. Hence, you can work around this by explicitly preventing the <svg> element from shrinking (with flex:none whose main effect in this case is to set flex-shrink:0)