Many computers have an On Screen Keyboard available. Try a web search.

Is this just with Firefox?

Start Firefox in Safe Mode {web Link} by holding down the <Shift>
(Mac=Options) key, and then starting Firefox. A small dialog should appear. Click Start In Safe Mode (not Refresh).

Is the problem still there?

Perhaps you could give more details and explain what does or does not work and what you are trying to do.

Are you for instance referring to characters that may be used in a website address, or typed in to a text area by Firefox. Or maybe you are asking about events javascript will recognise, or could it be a question about how a webpage is rendered.

See also:

• https://developer.mozilla.org/en-US/docs/Web/API/KeyboardEvent/keyCode#Value_of_keyCode