Firefox 2 on Windows, default font size – ok:

Firefox 2 on Windows, increased font size — the Find buttons show some strange artifacts:

Firefox 2 on Mac, default font size – ok:

Firefox 2 on Mac, increased font size — the Find buttons show the same strange artifacts as on Windows:

Firefox 2 on Mac, incredible increased font size — aha! This is how the UI is implemented. And I suppose Xing can also do better regarding accessibility: