User Agent: Mozilla/5.0 (Windows NT 6.1; WOW64; rv:45.0) Gecko/20100101 Firefox/45.0 Build ID: 20160124004010 Steps to reproduce: Used 'Inspect Element' on a page with a sourcemapped CSS file where the styles.css.map file had a HTTP 403 returned. You should be able to trigger this on the Crazy Domains website. Actual results: Mapped CSS line number does not display, files list in 'Style Editor' tab shows only a perpetual loading spinner. Clicking on source file line number link in 'Inspector' tab does nothing. Browser console has multiple instances of this: A promise chain failed to handle a rejection. Did you forget to '.catch', or did you forget to 'return'? See https://developer.mozilla.org/Mozilla/JavaScript_code_modules/Promise.jsm/Promise Date: Wed Jan 27 2016 09:07:57 GMT+0800 (W. Australia Standard Time) Full Message: SyntaxError: JSON.parse: unexpected character at line 1 column 1 of the JSON data Full Stack: SourceMapConsumer@resource://gre/modules/commonjs/toolkit/loader.js -> resource://devtools/shared/sourcemap/source-map.js:1281:20 StyleSheetActor<._fetchSourceMap/</map<@resource://gre/modules/commonjs/toolkit/loader.js -> resource://devtools/server/actors/stylesheets.js:753:21 Handler.prototype.process@resource://gre/modules/commonjs/toolkit/loader.js -> resource://gre/modules/Promise-backend.js:933:23 this.PromiseWalker.walkerLoop@resource://gre/modules/commonjs/toolkit/loader.js -> resource://gre/modules/Promise-backend.js:812:7 Promise*this.PromiseWalker.scheduleWalkerLoop@resource://gre/modules/commonjs/toolkit/loader.js -> resource://gre/modules/Promise-backend.js:743:11 this.PromiseWalker.schedulePromise@resource://gre/modules/commonjs/toolkit/loader.js -> resource://gre/modules/Promise-backend.js:775:7 this.PromiseWalker.completePromise@resource://gre/modules/commonjs/toolkit/loader.js -> resource://gre/modules/Promise-backend.js:710:7 mainThreadFetch/onResponse@resource://gre/modules/commonjs/toolkit/loader.js -> resource://devtools/shared/DevToolsUtils.js:642:7 NetUtil_asyncFetch/<.onStopRequest@resource://gre/modules/NetUtil.jsm:128:17 Expected results: Files list should show loaded files (minified sources) Source file line number link should switch to 'Style Editor' tab on correct file.

I believe this was fixed by adding the try/catch to _fetchSourceMap back in bug 1249888.