I suffered this problem yesterday too, but eventually tracked it down to Firefox: "Edit" > "Preferences" > "Advanced" > "General" > "Accessibility" > "Always use the cursor keys to navigate within pages" had become checked. If multiple Ubuntu developers are experiencing it, that suggests a bug in the Ubuntu firefox package.
I suffered this problem yesterday too, but eventually tracked it down to Firefox: "Edit" > "Preferences" > "Advanced" > "General" > "Accessibility" > "Always use the cursor keys to navigate within pages" had become checked. If multiple Ubuntu developers are experiencing it, that suggests a bug in the Ubuntu firefox package.