Start Firefox in Safe Mode {web link} by holding down the <Shift> (Mac=Options) key, and then starting Firefox.

A small dialog should appear. Click Start In Safe Mode (not Refresh). Is the problem still there?

What do these websites say about your location?

http://whatismyipaddress.com/ Under the map, look for Location Not Accurate?

http://browserleaks.com/geo

https://whatismyipaddress.com/geolocation

https://html5demos.com/geo/

You can try to rename prefs.js temporarily to see if that helps.

You can use the button on the "Help -> Troubleshooting Information" (about:support) page to go to the current Firefox profile folder or use the about:profiles page.

• Help -> Troubleshooting Information -> Profile Directory:
  Windows: Show Folder; Linux: Open Directory; Mac: Show in Finder
• http://kb.mozillazine.org/Profile_folder_-_Firefox

@FredMcD: as mentioned in original post, Safe Mode does not help. My geolocation is ok, though some providers think I'm in the wrong state, but still the same time zone. This is time from the browser though and not just what websites thinks (local Javascript will report wrong time).

@cor-el: Renaming prefs.js did not help. However, upon renaming it back, I lost a lot of settings, unfortunately (open tabs, tree style tab setup, layout, etc etc). So perhaps I am at the point where I just have to start fresh anyway.

Well after yet another reboot, the time zone in the (now somewhat screwed up) original profile is actually correct. Very strange problem.

Are you using a Proxy or VPN connection?