It is not entirely you. I too get a message to update to the latest version of Flash when I try to use the Comcast speed test; of course my version is the latest, but they offer an alternate HTML beta test site.
Two solutions: try using Firefox and/or download the Xfinity Streaming app on an iDevice (the latter works quite well; haven't tried the former).
On a Mac since 1984.
Currently: 24" M1 iMac, M2 Pro Mac mini with 27" BenQ monitor, M2 Macbook Air, MacOS 14.x; iPhones, iPods (yes, still) and iPads.
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