fix(css): resolve Firefox menu visibility with backdrop-filter fallback

[pzanella]

Related:

Fixes #1599 - Firefox Desktop and Mobile - Settings Menu not showing / correctly - Default Settings

Description:

Resolves menu invisibility in Firefox by disabling the problematic backdrop-filter property that causes rendering issues with positioned elements in Firefox's engine.

Problem:

• Settings and chapters menus were invisible in Firefox desktop browser
• Menus were present in DOM but not visually rendered
• Issue was specific to Firefox due to backdrop-filter: blur(4px) rendering bugs

Solution:

• Added Firefox-specific CSS rule using @-moz-document url-prefix()
• Disables backdrop-filter only in Firefox while preserving it in other browsers
• Maintains menu functionality without affecting visual design in Chrome/Safari/Edge

Technical Details:

• Uses browser-specific CSS targeting to isolate the fix
• Zero impact on other browsers (they ignore the @-moz-document rule)

Ready?

✅ Yes, this PR is ready for review. The fix has been tested and is production-ready.

Anything Else?

Testing:

• Verified menu visibility in Firefox
• Confirmed no visual regression in Chrome/Safari/Edge
• Tested both settings and chapters menus
• No JavaScript changes required

Browser Support:

• Firefox: ✅ Menus now visible
• Chrome/Safari/Edge: ✅

CSS-only solution:

• No unit tests required (static CSS rule)
• No breaking changes
• Minimal code change with maximum impact

Review Process:

For reviewers, please verify:

1. Code Review:

   • CSS syntax is correct and follows project conventions
   • @-moz-document url-prefix() targeting is appropriate

2. Manual Testing (if possible):

   • Open player in Firefox and click settings/chapters buttons
   • Verify menus are visible and functional

3. Cross-browser Compatibility:

   • Confirm other browsers ignore the Firefox-specific rule
   • Validate no console errors in any browser

Files Changed:

• packages/vidstack/styles/player/default/menus.css - added Firefox backdrop-filter fix