Switch on Num Lock to prevent Firefox from seeing the key presses as commands.

It works fine for me...

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

I have this same issue, even running in safe mode. I do have Num Lock on, but the issue isn't the numerals 0-9, it's the letters B, D, E, and F.

Alt+D switches focus to the address bar. If I type for example (hold Alt)+022d It will produce the character ȭ, but it will place it in the address bar.

Alt+(D, E, or F) will open the bookmarks menu, edit menu, and file menu respectively. If I type (hold Alt)+022(b, e, or f) which should produce ȫ, Ȯ, or ȯ respectively; the character will not be produced, and a little error "bing" will sound.

I'm on Windows 7, with Firefox 25.0.1