Current behavior
On Windows 10 (can't speak for 11), I can right-click a file → Open with → Choose Firefox → "Always use this app," and Firefox will open that one file type. But if I go to Settings -> Apps -> Default apps, and try to set my system-wide "Photo viewer" or "Video player," Firefox is not listed at all. I have to manually associate every single extension (.jpg, .png, .gif, .webp, .svg, .mp4, etc.) one by one.
What I want
Have Firefox register itself with Windows as a proper handler for common image and video types, so it shows up in the Default Apps picker.