When uploading a large file (say 1GB) to Alfresco using the html5-uploader in Share (either the 'drag & drop'-version or the 'Upload'-button in the DocLib), Firefox 30 cannot load any new pages until the upload finishes.
You can open a new tab, or click a link in a current, but nothing loads.
Also, when canceling the upload, it actually doesn't get cancelled and keeps on going till it's finished.
Exiting Firefox doesn't stop this, at the upload keeps the Firefox-process running in the background till you manually kill it.
After some searching, this might have something to with how the uploader is implementerd. See the Firefox-section on this page: http://new-bamboo.co.uk/blog/2010/07/30/html5-powered-ajax-file-uploads