popups.css: Add fix for #65
Based on user0's work (https://codeberg.org/user0/mobile-config-firefox/src/commit/f2381fd45eeef10ffb173a56f8106e6d68b981be/src/userChrome/06_popups.css#L85-L89), I've implemented a slightly different fix for this menu:
- max-width only, because that's enough to fix the menu
- 100% instead of 100vw, as that's less in most cases, but enough.