This issue can be caused by an old bitmap version of the Helvetica or Geneva font or (bitmap) fonts that Firefox can't display in that size.<br /> Firefox can't display some old bitmap fonts in a larger size and displays gibberish instead.<br /> You can test that by zooming out (View > Zoom > Zoom Out, Ctrl -) to make the text smaller. Uninstall (remove) all variants of that not working font to make Firefox use another font or see if you can find a True type version that doesn't show the problem.