Hello,

what character encoding is set?

1. Go to View > Character Encoding.
2. It should be set to UTF-8.

Kind regards, Tobbi

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. Firefox can't display some old bitmap fonts in a larger size and displays gibberish instead. 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.