Plex offers the convenience of accessing the same Plex content you are used to in their Plex Web Player direct from any supported browser. (Firefox, Brave, Chrome, Safari, etc). Open your browser of choice, go to Plex.tv, and sign into your Plex account. Once signed in, the “OPEN PLEX” button will appear. Click it to launch Plex in your current browser.
Once open in your browser, find the settings icon in the upper right-hand portion of the screen
Once in settings, find the section on the left called Plex Media Player and select Quality. Then using the drop-down menu, select Maximum.