I'm not wiping my Firefox Cookies, Cache, or History for your sake, but I did the rest and with no obvious change. I am using a single tab of Chrome for Pandora now. Songs no longer play simultaneously, but at the same rate, the media play/pause key will just stop working - until I reboot Chrome. So this is all probably different browser implementations of the media key and the incompatibilities therein. I'd love to just use a PC app actually built for Pandora (I was very happy with Elpis for years - functioning media keys and song preview popups), OR you could put some work into the official Pandora desktop app and fix the media keys....
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