I have encountered the same problem with entering either user or password on a number of sites including Mobile gmail(m.gmail.com),but not
www.gmail.com, even m.mobileread.com but not
www.mobileread.com, before and after the firmware update. I've only noticed it on mobile versions of sites, or sites that redirect kindles automatically to their mobile version. As a temporary fix I go through Skweezer.com or Google transcode at google.com/gwt/n