Looks like the glass should lift up, allowing you to grab and pull down on the trim ring. Usually straight down. Some trim I've seen, just one side flops down - and then the glass falls out and breaks. Do be careful.

Some old ones take a bit of a tug to get the spring started slipping through a groove.

If you've replaced the fixture it is a mute point. If you couldn't open the fisture, how did you get it down? The glass clobe is locked in place by three nubs. Usually if you press up lightly and turn counter-clockwise, the globe will come down.

From seeing other similar fixtures, the glass is molded with a ledge or ramp that catches on protrusions in the base. You sort of unscrew it but it may only rotate a bit to disengage. It may feel stuck but if you unscrew with more confidence it should rotate and come off.

Thanks to all of you! I found it was the "twist the globe" style. The frame wasn't tight and moved too, but I kept at it and it twisted loose. I was just about to put a plastic bag over it and bash it with a hammer, so I REALLY appreciate your help! --Kris