You may have accidentally set your language preferences to something else through a hot-key combination. Try following the steps here:

Control Panel > Regional and Language Options > Text services and input languages > Details (Key Settings)

If it isn't that, be sure your Firefox version is up to date.

If you have the latest version, then reset Firefox by going to:

Help > Troubleshooting information > Reset Firefox...

Please give us an update to see if any of these things fixed your problem. Thanks! =)

In April, a similar symptom was traced to the ZoneAlarm "anti-keylogger" feature: When i hit a key to type it cycles through every key on the keyboard. If you use ZoneAlarm, you may want to check into that.

Thanks. That worked