When I open the Reader View in Firefox, it adds some disturbing static, whenever sound is played (e.g. youtube, music player, system sounds from the wingpanel...).
Neither exiting the Reader View nor closing Firefox will fix this, but logging out and in again does.
I even tried capturing it with audacity and this guide, but surprise, the static will not appear in the recording.
I'm not sure if this is an issue of Firefox or elementary OS, but I figured, it might as well be one of elementary, as closing Firefox does not fix it.