Thanks to the author.

Th extension does what it says, use the 'Fixed Location' option if high precision location is not important for you. Read the FAQ. It does not change IP Location though, only Broswer Gelocation is changed (even without using GPS).

Despite claiming to be configurable on a per-site basis, I was unable to determine how to do this (extensive searching). And, if the extension's icon is supposed to be visible, it is not on my system (FF 121.0), and while I can find it in the overflow "Extensions" button on the toolbar, it is greyed-out and unresponsive. I uninstalled it after an hour, as I need its capabilities for ONE site, not all sites, and it does not appear to have this capability.

The extension is working perfectly what I wanted.

Pros:
- does what it is supposed to do
- simple
- tiny, not bloated
- uses OSM/Leaflet for settings instead Google Maps, so additional + for privacy

Cons:
- lack of per site settings (my reason of 4 instead 5 stars)
- doesn't give button to turn on/off extension (the only way is via Fx built-in mechanisms)

This option seems making confuse between the fixed location and privacy level, causing wrong geo location..

It works - more or less - but the extension icon is not shown in the Firefox toolbar. Besides, I'd love it if I could have a per-site location. Some sites I don't want to hide my location from, some I do. It's hard to remember that before visiting the site.
We need more customization!

Hit or miss, though I don't think that is the extension's fault. I use a VPN, and that often means that sites display things based on my IP's location, but inappropriate for my actually location. I used the extension to set a fixed location and some sites respect it and others do not. For instance, lowes.com displays the store using the location of my VPN IP address on the other side of the state, and homedepot.com uses the fixed location from the extension. I think you just need to keep in mind that different sites use different techniques to derive your location. In my case, I want the security of a VPN with the convenience of sites knowing roughly, but not exactly, when I am. That does not seem to be 100% possible with this extension, but I'll use it for a while to see if it's good enough.