When viewing a page with debug=1 (e.g. https://en.wikipedia.org/wiki/The_Fighting_Temeraire?debug=1), Firefox prints errors to the browser console about broken source maps for OOUI modules:
Source map error: request failed with status 400 Resource URL: https://en.wikipedia.org/w/resources/lib/oojs-ui/oojs-ui-core.js?db4bf Source Map URL: oojs-ui-core.js.map
It's trying to load the source map from https://en.wikipedia.org/w/resources/lib/oojs-ui/oojs-ui-core.js.map, which responds with only the text Invalid file type. Presumably there is a whitelist of file types we serve in production, and .map is not on it.
The warnings are harmless, but unprofessional, and they make our users uneasy when we ask them to load a page with debug=true and copy the console output :)
The Popups extension also generates source maps and works around this by using .json extension for them, e.g. https://en.wikipedia.org/w/extensions/Popups/resources/dist/index.js.json.