How do I investigate a "style sheet could not be loaded" message in Firefox?
<p>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.</p>

<p><strong>Update</strong></p>

<p>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.</p>

<p><strong>Another Update</strong></p>

<p>This error bar comes and goes, it is not consistent for a particular page.</p>