Open in browser as Other... does not work

[pmatas]

The Opening dialog pops up and I choose "Open in browser as" "Other...". An empty text box appears under the drop-down and I fill it with a mime type (I have tried "text/plain", "text/plain;charset=utf-8" and "text/html"). I hit OK, but nothing happens, the Opening dialog stays open and responsive.
Adding the mime type to the pref extensions.openinbrowser.additional_mimes works as expected.

WinXP Prof CZ SP3 32-bit, Firefox 49.0.2, Open in Browser 1.17