Browser zoom level: Check if the browser zoom level is set to something other than 100%. If so, try resetting the zoom level to 100% by pressing the Ctrl+0 (Windows) or Command+0 (Mac) keyboard shortcut. Video player resolution: Some video players may default to a lower resolution, resulting in blurry playback. Check the video player settings to ensure that the resolution is set to the highest possible quality. Hardware acceleration: Firefox uses hardware acceleration to improve the performance of graphics-heavy tasks such as video playback. However, if the hardware acceleration feature is not working properly, it can result in half of the screen being blurred. To disable hardware acceleration, go to the Firefox menu > Preferences > General > Performance, and uncheck the "Use recommended performance settings" option. Then uncheck the "Use hardware acceleration when available" option. Outdated graphics drivers: Outdated graphics drivers can also cause video playback issues, including half of the screen being blurred. Check your computer's manufacturer website for the latest graphics drivers, and update them accordingly. Check if this helps.