Contents.css not applied in Firefox

I am using ckeditor 4. I have set some cutsom styles in the 'contents.css' file, and everything works fine in chrome. In Firefox (version 31.0) instead, even though the file seems to be there and correctly loaded into the DOM (I have inspected the source code with Firebug and the css file is there and the path is correct cos I have opened it in a separate tab), the styles are not applied to the contents of the IFrame window of the editor. If I inspect with firebug the single DOM nodes inside the IFrame (like 'body' foe example) there are no styles from the contents.css file. If I expand the css file node from inside Firebug, instead of showing the css' contents, it says 'Reload the page to get source for: <pathOfMyFile>'. I have read somewhere on the internet there might be some cache issue, so I had appended a random generated number as a query string parameter (that means I have tried setting the path of contents.css manually in the config file) but it doesn't work either. Any ideas?