Thanks Yoc, I just retested the site on a Windows 7 SP 1 laptop using Firefox 90.0.1 64-Bit and the Expressa Deco Noir Theatre videos thankfully looked and performed as expected. I'm not to sure what could be the specific issue going on with the rendering in Firefox on your side. I am glad Brave was able to display everything for you in all its intended glory.
If the cache purge did not resolve it in Firefox and you are running its latest version, then perhaps an errant plug-in may be interfering? Do you still have Adobe Flash installed on your Windows laptop? If so, it should be uninstalled completely as it has been decommissioned back at the end of last year. Vimeo, on which the videos are hosted, specifically states they no longer support Flash. They additionally state that Vimeo requires browsers that are able to decode H.264 videos in players with HTML5 functionality.
I'm not sure if this helps, but I'm hoping so. If it's able to display as intended on Brave, it appears there's something awry with Firefox on your PC. It also rules out anything system related, otherwise the blurry video previews would exhibit the same behaviour in Brave.
Best of fortunes to you,
- TGM