Fixture is defined in the LEXICO Dictionary as:
A piece of equipment or furniture which is fixed in position in a building or vehicle.
Here're a couple examples from the dictionary:
Cathedral ceilings, antique light fixtures and furniture that looked as though it had come straight from the twenties.
During assembly, the glass is first positioned in a fixture with several sensors around the sunroof.
So, fixture seems to be a count-noun, albeit usually used in the plural.
But as its definition indicates, fixture is similar in meaning to such mass nouns as equipment and furniture in that it encompasses different objects with different shapes and functions that can be subsumed under a single category. And I thought this kind of special meaning was the very reason for treating equipment and furniture as a mass noun, so I was wondering how fixture is treated as not a mass noun but a count noun.
Is there anything with fixture that distinguishes it from the equipment/ furniture types of mass nouns that makes it a count noun?