Steps to reproduce:
Search for something.
Click on the region selector thing, which opens the region dropdown.
Click on a region from the dropdown, or on the region search box.
Clicking on a region from the dropdown should... do something? Apply that setting and re-do the search with it, I assume. That seems to be what it does on Chrome. It does not do anything on Firefox.
Clicking on the region search box highlights it for a split second, but then immediately loses focus, so attempting to type on it will instead do whatever the key shortcuts for those keys do. It should instead let you type on it.
Firefox 97.0.1 / macOS 10.14.6 / Active Kagi region is Spain