Start Firefox in Safe Mode to check if one of the extensions (Firefox/Tools > Add-ons > Extensions) or if hardware acceleration is causing the problem (switch to the DEFAULT theme: Firefox/Tools > Add-ons > Appearance).

To Enable SafeMode

• You can open Firefox 4.0+ in Safe Mode by holding the Shift key when you open the Firefox desktop or Start menu shortcut.
• Or open the Help menu and click on the Restart with Add-ons Disabled... menu item while Firefox is running.
• Once you get the pop-up, just select "'Start in Safe Mode"

If it works in Safe Mode and in normal mode with all extensions (Tools > Add-ons > Extensions) disabled then try to find which extension is causing it by enabling one extension at a time until the problem reappears.

Close and restart Firefox after each change via "Firefox > Exit" (Windows: Firefox/File > Exit; Mac: "Firefox > Quit Firefox"; Linux: "Firefox/File > Quit")

• https://support.mozilla.org/en-US/kb/troubleshoot-firefox-issues-using-safe-mode
• https://support.mozilla.org/kb/Safe+Mode
• https://support.mozilla.org/kb/Troubleshooting+extensions+and+themes

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If its not working

Create a new profile as a test to check if your current profile is causing the problems. See "Creating a profile":

• https://support.mozilla.org/kb/profile-manager-create-and-remove-firefox-profiles
• http://kb.mozillazine.org/Standard_diagnostic_-_Firefox#Profile_issues

Profile Backup and Restore

• http://kb.mozillazine.org/Profile_backup
• https://support.mozilla.org/en-US/kb/back-and-restore-information-firefox-profiles
• http://kb.mozillazine.org/Transferring_data_to_a_new_profile_-_Firefox

What kind of letters do you get instead of the ones that you press?

Are this accented letters or entirely different?

Are you sure that you use the correct keyboard layout when you are in Firefox and haven't switched to another layout by accident?

• http://windows.microsoft.com/en-us/windows-8/language

I had this problem and fixed it. LEFT SHIFT + LEFT ALT is a hotkey command that switches your typed language, so try hitting those two buttons together.

This probably happened because I hit that by accident, and in my Windows 7 Region and Language settings I have "Japanese" set as the language for programs that don't support Unicode (this is for playing some Japanese games). ｶｸﾆﾄ ﾆﾄ ﾃｸﾁｶ ﾓﾝ ｶｲｻｶ ﾘﾗﾗﾉｲｼ ﾘﾆﾉｲﾇ <<< This is what my text looked like!

The default key combination to rotate the layout is the Ctrl+Shift or Alt+Shift combination that is used in Firefox for menu items, so it is quite possible to change the layout accidentally.

To avoid an unintentional switch, assign a specific key sequence (Alt/Ctrl+Shift+number) to select keyboard layouts and remove the key combination to rotate layouts (Alt+Shift or Ctrl+Shift)