Yeah, this seems to happen if your browser is started from zero-k initially. I can recreate this by:
1. Close all browsers.
2. Launch Zero-k
3. Click "Home" in zero-k -- this launches an instance of firefox as "/usr/lib/firefox/firefox https://zero-k.info/?asmallcake="
4. Close Zero-k. The firefox instance will still be running with "zero-k" in its title.
5. Attempt to launch Zero-k -- it will fail.
6. Close firefox.
7. Relaunch Zero-K -- it will succeed this time.
This should be an easy fix by the devs -- it's almost certainly just looking for "zero-k" in the processes list.