The diffuser is held onto the bulb's base by four snap-lock plastic tabs, which also have a dab of silicone adhesive on them to prevent the diffuser from rattling.

The tabs are just to the right of the small Phillips-head screws you can see in the small space between the diffuser and base.

Insert a strong screwdriver or similar prying tool to bend the plastic tabs. You'll need to bend at least two at once, and then insert something to hold open the one side while you work on the other.