@Justinrleung mentioned to me on Discord that the code editor fails to load properly on English Wiktionary in modules (and JavaScript and CSS pages). In the console there's an error mentioning wikiEditor in the backtrace. The issue starts showing up in wmf.9.
I looked at the prettified source code in the Firefox developer tools and did a code search, and the offending line seems to be in jquery.wikiEditor.js on line 551. The variable module is undefined. The module variable was renamed to m in this commit on May 23, but one use of module was left behind, causing the error. Looks like the solution's as simple as fixing that variable name.