According to a blog written by University of Pennsylvania linguistics professor Mark Liberman, there seems to be some disagreement over whether the term "glass slippers" is a mistranslation of the French word for "squirrel fur," ("verre" for glass and "vair" for squirrel fur), with Liberman concluding on the side of mistranslation and asserting the slippers were indeed made of squirrel fur rather than glass. The very first published mention of a glass slipper in the Cinderella story was printed in 1697 in a volume of French folk tales written by Charles Perrault. At the time, squirrel fur was considered a luxury item, though its use was somewhat outdated and may have led to a misquote on the part of the printer, who, Liberman conjectures, may have used the word "verre" instead of "vair."
Other definitive versions of the Cinderella story, including the Brothers Grimm version, which was published in 1812 as part of "Grimm's Fairy Tales," use a different version of the slipper. In the Grimm's version, the slipper is made from gold, not glass or fur. Out of all three materials, fur may be the most logical and comfortable for a night of dancing, but the truth seems to be that we will never know what Perrault meant by "verre."