Ok about 4-5 days ago now all of a sudden file uploading stopped working on FireFox for most websites, including ebay.co.uk when listing a new product (both basic and advanced), my own website when hosted on one server, but oddly not another, and on example sites like http://blueimp.github.io/jQuery-File-Upload/ If you drop a file it uploads to between 325kb to 350kb then just hangs indefinitely before getting an XHR return code 0 (if debugging to see the error) for AJAX file uploaders. I have tested this on 4 machines, 3 operating systems, 2 virtual servers and friends in UK, USA and Holland are all having this issue as of about 4-5 days ago. Has something updated in FireFox since then to cause this, or perhaps a Windows Update has screwed something up that we all have installed? The issue also seems to have affected IE. The error doesn't happen on machines I have running Windows Server editions (2008 R2 and 2012) then can use FireFox or IE and upload to any of those sites with the issue. Also to upload to the sites that crash, if the exact same file upload site is hosted on Windows Server 2012 (might just be coincidence though) it works for all clients. I know this because I made one of the sites that has this same issue and I have it hosted on a 2008 server and 2012, and the 2012 doesn't have any issue with clients. However the fact that eBay is suffering this same bug on says its pretty big, and everyone I can find to test this has the issue also. And I've tried FireFox in safe mode too and still same errors.