How do I get the glass off this ceiling fixture
Three flat spring arms extend down from the base. They have bends so that they can hook the glass once it is fully in place. Pull the glass down a bit from one side only. Slide a butter knife around until you find one of the spring arms. Then either tug down or use the butter knife to push the spring inward while you tug.
When you re-install the glass, hook 2 of the springs into the glass and use your fingers to push the third arm in as you raise the glass dome.