How to loosen glass bulb cover from metal base?
Try squirting some sort of grease such as WD-40, or even more natural and thicker oils like olive oil into the part where the glass cage touches the inside metal.

Make sure to hold the fixture downward in the way that you pictured and try to minimize getting grease on the part of the glass which you grab.

Apply a damp cloth to the glass cage and fan the area to cool and thus take some of the pressure off of the thread in the metal. Switch between the damp cloth and setting it out in the sun to compress and decompress the glass, all the while attempting to turn it back and forth.

Hope this helps!