Ollie,

This is a pure SWAG on my part, but I've encountered so many bizarre problems caused by cache corruption that the first thing I'd recommend is that you clear all browser cache in Firefox. If you use CCleaner it will work (and a bit faster) or you can follow the instructions on the Firefox Clear Cache <https://support.mozilla.org/en-US/kb/how-clear-firefox-cache?redirectlocale=en-US&redirectslug=How+to+clear+the+cache> page.

Try that and see if it resolves the problem. If it doesn't at least we can eliminate cache corruption as the root cause.

Brian