Created attachment 8867558 [details] 12May2017_TestCase_3_Step_5.png User Agent: Mozilla/5.0 (Windows NT 6.3; rv:36.0) Gecko/20100101 Firefox/36.04 Build ID: 20170403072723 Steps to reproduce: 1. Launch Firefox. 2. Go to any webstie e.g: http://edition.cnn.com/ 3. Right click on the page and choose Inspect Element (Q) from the context menu. 4. Select Network tab from the Developers tools. 5. Open Details view by clicking Show request details button from the Dev tools upper right side. Actual results: Preview is not displayed if the content type is HTML after clicking Show request details button from the Dev tools. Expected results: Details view is properly open showing Headers (in focus), Cookies, Params, Response, Timings, Security (only for secure pages) and Preview (only if the content type is HTML).
Works for me on 54.0b10 (20170522172523) using Windows 10 x64, Ubuntu 14.04 x86 and Mac OS X 10.11.6. The Preview tab is displayed if the request type is HTML. Feel free to reopen if it still occurs for you or if you find proper steps to reproduce.