How can I fix WebStorm warning "Unresolved function or method" for "require" (Firefox Add-on SDK)
Do you mean that require() is not resolved? You need to either add require.js to your project or enable Node.js Globals predefined library in Settings/Languages and Frameworks/JavaScript/Libraries.
(Edited settings path by @yurik)
In WebStorm 2016.x-2017.x: make sure that the Node.js Core library is enabled in Settings (Preferences) | Languages & Frameworks | Node.js and NPM
In IntelliJ 2018.3.2+ go to Settings (Preferences) | Languages & Frameworks | Node.js and NPM and enable Coding assistance for Node.js
Webstorm 11 and 2016.2.3
Enable Node.js Core library in Webstorm settings.
After spending an hour trying to get this to work using all solutions found online, this finally did the trick!
File -> Invalidate Caches/Restart...