There is a bug in Firefox and/or in how Fedora packages it, that prevents installing web apps from Mozilla Marketplace or any other source. It’s because Firefox in Fedora is shipped as Xulrunner application. This makes some assumptions about path wrong for Firefox.
You can easily work around this using couple of symlinks. I provided the solution in a comment to bug report but I paste it here too:
ln -s /usr/lib64/firefox/webapprt-stub /usr/lib64/firefox/xulrunner/ ln -s /usr/lib64/firefox/webapprt /usr/lib64/firefox/xulrunner/ ln -s /usr/lib64/firefox/application.ini /usr/lib64/firefox/xulrunner/