The Stadt- und Industriemuseum sits in a former judge's house in the Altstadt and complements the literary and legal museums by telling the longer economic story of the city: the medieval iron mines of the Lahn-Dill region, the imperial court period, and most prominently the optical industry that grew up around Carl Kellner and Ernst Leitz in the 19th century.
The Leica section is good, with examples of the early prototypes, the M-series cameras, and the microscopes that originally made the company. If you only plan to visit one factory-themed museum, the Ernst Leitz Museum at Leitz-Park is more comprehensive — but the city museum places the optical story in its wider Wetzlar context.