How do I investigate a "style sheet could not be loaded" message in Firefox? This message appears as a red bar below the page contents and above the developer tools. How do I find out what file the browser is referring to? I'm running version 46.0 on Linux Mint 17.3.
If I look at the console of the developer's tools in Firefox, it shows all the css files and says "HTTP/1.1 200 OK" against each file.
This error bar comes and goes, it is not consistent for a particular page.
This happens almost always when CSS is gziped, but server returns Content-Length of not compressed resource. Seen this happen when using mod_deflate with mod_fastcgi. This is server side bug, not firefox.