If you have to climb higher than the second-highest rung to reach the fixture, you need a taller ladder.
Stop tightening the diffuser's finial as soon as you encounter resistance.
Even if you've fitted all your light fixtures with long-lasting LED bulbs, you'll have to change the bulbs at some point. And when the fixture is a flush-mounted one, the procedure can be slightly hazardous. Because it's usually high on the wall or on the ceiling, you need to climb on something, and you could fall. Moreover, you need to remove the glass diffuser to access the bulb, and that introduces the possibility of dropping the diffuser and breaking it. It's important to work safely -- always use a ladder or chair that allows you to work comfortably without stretching.
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Turn off the switch controlling the fixture, if it still has a working bulb, and leave it off for 10 minutes to allow the glass dome to cool.
Set up a stepladder under the light fixture, making sure that all four feet are firmly on the floor and the spreaders are fully down and locked. A 4-foot ladder is tall enough to enable most people to reach the ceiling in rooms with standard 8-foot ceilings.
Climb the ladder high enough to reach the fixture without stretching. Support the glass diffuser with one hand while you unscrew the center finial and hexnut or diffuser cap with the other. Place these and any washer if present on the ladder shelf or top step.
Remove the diffuser, climb down the ladder and put the diffuser on a table; alternatively, place the diffuser on the ladder's shelf. Climb back up the ladder with the replacement bulb, then unscrew the burnt-out bulb from the fixture and screw in the new one.
Climb back down the ladder, put the old bulb in a safe place and retrieve the diffuser. Climb back up and hold it on the fixture while you replace the screws to hold it. If the fixture has three screws placed around the rim, tighten them incrementally in turn to keep the diffuser centered.