I first noticed the issue when I attempted to open Pandora.com in the Steam in-game browser, and got this message: "In order to use Pandora internet radio, please install Adobe Flash (v.10 or later)." As I am running Win10(64bit), it should be already built in; indeed, it works in Chrome, Firefox, Edge, and even IE. This help page from Valve is unfortunately out of date, and installing all of the most current versions of Flash - standard, other versions, and by direct download - has not made any apparent change. (Yes, I have tried turning it off and on again.)
It should be noted that while this is a relatively recent issue for me, others have had similar issues in the past (one post I encountered was from over 8 years ago). That being said, none of the various solutions I've found are still workable - many of the links are dead or the content that was mentioned has since been removed or altered. Any help with an updated solution for the current version of Steam (Steam package version: 1484790260 or later) would be greatly appreciated.