This is what it says about codec for Firefox.

"HTML5 audio and video in Firefox" https://support.mozilla.org/en-US/kb/html5-audio-and-video-firefox

On Mac there is currently no support for HEVC videos, on Windows there is support via WMF in about:config controlled by media.wmf.hevc.enabled.

Unfortunately, I use a Mac. I love Firefox as my browser. It is disappointing that Apple & Microsoft addressed the issue, but not Mozilla. Thank you for your quick response.

Thank you Markwarner 22 for your quick reply. I wish I was more technical to delve into the workarounds suggested in the support article. Unfortunately as I read the article, I would have to do too many things to use the suggested method in the embedded software that Arlo uses. It is just simpler to open the app in the Safari browser. I wonder how many apps out there have the same problem with HEVC in Firefox?

Your only other choices are 3rd party codec software that may or may not help with the playback issues but you have to ask those sites if you want to use their codec on your iOS to insure it works and is safe to use as well. Also check with your Cam manufacture support since this is their hardware and should know more it's software support.

if you want to use their codec on your iOS to insure it works

You mean macOS as the iOS is mobile OS for iPhones and early iPads. As this is not the first time you thought Mac uses iOS as the OS. https://en.wikipedia.org/wiki/MacOS https://en.wikipedia.org/wiki/IOS