I had a similar a few years ago. Give the glass a sharp hit with the heel of your palm whilst holding the opposite side.
Mine had a rubber sealing ring on the inside, which helped make it weatherproof, but which had caused it to stick to the base.
If you've already removed the cage, the glass MAY just be stuck to fitting. There may be a rubber gasket that's making everything a bit sticky. Give it a gentle pull.
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