The text field above should be automatically highlighted, similar to the "paste link" dialog in Google Maps.
But it is not selected in Firefox, at least on Linux, so it isn't pasteable.
To verify that, try pasting it somewhere with the middle mouse button
and verify that "Hello, world" is not pasted;
or install the xsel program and run this command from a shell:
echo -n primary:; xsel -p; echo; echo -n secondary: ; xsel -s; echo; echo -n clipboard:; xsel -b; echo
Clicking or double-clicking in the page won't help.
The problem only exists in Firefox, not in Chromium or Midori, so I've filed it as Bug 926257 - Can't select link text from fields on google maps (and some other pages).