Co. was used by German companies but it could also be a translation - though Ltd. would be a more likely translation in UK English. Also companies would put translations of the company name on the cover of catalogues for any overseas office. There are instances when a writer has used the English subsidiary rather than the parent German name, especially where they could not type ü for some reason.
Looking for Müller is a bit like looking for Miller, a common name anywhere that made bread. Ivo's book covers glassworks and these are as likely to be a retailer or a wholesaler.
Plenty of hits in Glass Study, all refiners e.g.
Müller Sachs, Heinrich. Glasbläserei. Steinach
Müller, Gebr., Schmalenbuche bei Neuhaus am Rennsteig
Müller, Werner, Schmalenbuche bei Neuhaus am Rennsteig
Müller, Bruno & Co. TA: Müller Kompagnie. Bleikristalle mit Hochglanzpolitur. Petersdorf i. Riesengebirge
Müller & Co. Glasfabrik, Piesau
Müller & Kühnert. Piesau.
Müller, Ferdinand. Glasmalerei. Quedlinburg
Müller & Co. Neuhaus am Rennweg
Müller, F. G. Glaswarenfabrik. Oberweißbach
Müller & Co., Rudolf, G. m. b. H. Ilmenau
Lots of Müller in Lauscha... mostly doing eyes and lampwork or making the rods/tubing for lampwork. Lots of other Müller but very unlikely candidates.
Note that while the Glass-Study may be comprehensive it does not always enrich the data as a finished study like Ivo's Glass fact file does. It provides one or more snapshots in time only, not period of operation, and is also subject to original errors, even intentional ones. So on their own, the above data should be treated with caution.