I have done extensive testing on this subject with Thorin here: timezone problems · Issue #9 · anthonysgro/geospoof · GitHub
We have gotten Firefox fingerprinting about as tight as possible for a web extension that spoofs your location. All practical methods to detect lies in the browser are patched (to my knowledge).
That being said, Firefox does expose a couple of extremely niche OS-level apis that do expose your system date/time. There is nothing any extension can do about this. Realistically, the vast majority of trackers and websites are not using these to determine your timezone. However, it IS possible, therefore I cannot recommend this extension for those with extreme threat models. If you are worrying about this, you should be using TOR or Mullvad instead.
One workaround for the above is to set your system OS date time to the GeoSpoof timezone. This actually completely passes the Arkenfox fingerprinting test suite here: TZP
Otherwise, this is about as good as you can spoof location data on Firefox, Chrome, etc.