Strange emacs buffer instead of capture buffer

Hi,

This issue was found in [this](https://github.com/kuanyui/copy-as-org-mode/issues/1) issue of another project and moved to here in order to let it be found by others as well.

Hard to find a good title for my issue. Please rename it if there is a more suitable one.

I'm on Windows 10 with Emacs 26.0.90 and Org 9.1.6 (=most current git/maint).

When I invoke the org-protocol javascript bookmarklet on a web page (not an empty tab or the about page of Firefox), I get an open new Emacs buffer for `c:/Program Files (x86)/Mozilla Firefox/org-protocol:/capture-html/?template=l&url=about%%3Ablank&title=%%5Buntitled%%20page%%5D&body=`

Mysterious to me 😉

Not only that there is "about:" in the URL and so forth, there is no body. The basic communication from Firefox to Emacs seems to work though.

What should I check?