I use the Safari browser, and none of the text editor pop-up windows work.
There is a known bug with using Safari and the rich-text editor.
If you use Safari, you have one of two options: (a) disable the
rich-text editor and type the HTML directly (by clicking the "disable rich-text" link below the text entry field), or (b) use an alternate
browser such as Firefox (recommended) or Camino.