Roblox was broken on Deck and linux by the addition of a new anti-cheat
Afaik, it wasn’t broken. The Roblox devs put in extra work to block Linux users from playing because according to them “the majority of cheaters are on Linux”. Who knows if that’s true
Install Video DownloadHelper for Firefox. Open whatever you want to watch and copy it’s url from Video DownloadHelper. Now, either open that url in mpv (might require yt-dlp) or download it (I’d also recommend yt-dlp for this, since DownloadHelper sometimes embeds watermarks) and watch wherever