I cannot use iCloud.com in Firefox at all. Starting from the login prompt there are many issues. From the e-mail field, you cannot use Tab to move to password field, only… (tuilleadh eolais)
This occurs on Windows 8 64bit and Firefox 18/19 Beta. I have gone through many troubleshooting steps and it has made no difference in the slightest. I have cleared all caches several times, from inside Firefox and through 3rd party utilities. Tested in Safe Mode to no avail. Manually removed several add-ons, checked all add-ons are fully up-to-date (as always), and reset all Firefox settings to default (except Hardware Acceleration that causes graphics issues in 8 - does not help iCloud.com enabled, though). It works fully in all other browsers with similar add-ons without issue. I have not done a full, complete reset and profile removal - that is next. However, I have a gut feeling that is not the issue at hand. I performed a clean install of Windows 8 and a clean install of Firefox less than 2 weeks ago.