Pixelated text suddenly on 2011 21.5" iMac

Hi

I've just this minute noticed that the text in the Firefox web browser and some other programs on my 2011 21.5" iMac is pixelated and some letters in words are lighter than others (in the image below you can see that the i of 'in' and i of 'Safari' are lighter than the other letters).

I've tried going into System Preferences > General and unticking and ticking 'Use LCD font smoothing when available' but this hasn't made a difference?

Does anyone know what the problem might be? I've had my iMac since new.

I already wear reading glasses, so I'm sure it's not my eyes.

Thanks

Paul