User Agent: Mozilla/5.0 (X11; Linux x86_64; rv:55.0) Gecko/20100101 Firefox/55.0 Build ID: 20170619141703 Steps to reproduce: On de lastversion on firefox developer edition when I look in the network tab, and inspect an request, it doesn't show the preview tab anymore. Instead it show's a stack trace tab. Actual results: it doesn't show the preview tab anymore. Instead it show's a stack trace tab. Expected results: it should have showed a preview tab, where I can preview the html response.

This is really important to XHR work as there's no other way to visually see errors that come up while processing requests that are related to conflicts with data. Ideally this would be part of the console as well like it was in firebug also.

So, I think you are talking about HTML preview tab The preview will be part of the Response tab see also bug 1353319 I am closing this one, bug feel free to reopen if needed. Thanks! Honza