It's not a fluid. Fuck Yeah Fluid Dynamics debunks the myth that because glass is an amorphous solid, over a prolonged period of time it can behave as a slowly-moving fluid.
The observation that old panes of glass tend to be thicker at the bottom is usually used as evidence that glass flows over the centuries, but this assumes that the glass was flat to begin with. However, glassblowers at the time usually made panes by spinning molten glass to create a round, mostly even flat, which was then cut to fit. Although spinning made the glass mostly flat, the edges of the disc tended to be thinner. When installed, the glass was typically placed thicker side down for stability purposes. One researcher even calculated the time period necessary for glass to flow and deform at ordinary temperatures as 10^32 years—longer than the age of the universe.The most extensive discussion I've found of this subject is at this link.
Photo - 12th century window at Chateau de Beynac. Credit Vincent van der Pas.