In reply to Keiko.

Hi Keiko,
If you’re using Windows High Contrast, you can’t see selected text in Firefox due to a combination of a Firefox bug and the fact that some parts of the editor use a custom CSS colour for the selection highlight (besides, Firefox also doesn’t change links, though it seems maybe that can be set locally on your Firefox to work better: https://bugzilla.mozilla.org/show_bug.cgi?id=697836)

You’ll want to use Edge browser instead.