Please see the attached image.

I can see that site ok from the URL. It is only a few headings that are incorrect from your screenshot, try clearing the browser cache.

• Tools -> Clear Recent History : time = everything, [cache]
• if necessary also clear all cookies for that site.
• see How to clear the Firefox cache

Tools -> Preferences -> Content -> Default font

Try out different fonts and sizes...

Arial could help!

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.
There have also been fonts with a Chinese name reported that identify themselves as Helvetica, so check that as well.

Thanks cor-el, Your solution only solve this url, but it can't solve other website, although I change font default of FF.

I do not know whether it helps but I can read the webpage you took a screengrab of. I assume the language is intentionally none English, and note you manage the mathematical calculus, or whatever.

Your screengrab contains numerous placeholders, I see the intended characters so I can for instance see correctly

• header: Thích Học Toán
• link anchor: Euler và sự vô hạn của tập các số nguyên tố,

I note I am using an English version of firefox & XP, and viewing in firefox as Unicode UTF-8 with auto detect off. I do not know but if you can set up firefox to read this post without placeholders appearing would it then read the website using the same settings.

Thank Jonhn99 for your reply, how to setting placeholders appearing?

That is a missing font for the 1E00 plane

• http://en.wikipedia.org/wiki/Latin_extended_additional
• http://en.wikibooks.org/wiki/Unicode/Character_reference/1000-1FFF

You will have to install a font that covers missing ranges that you want to display properly.

See also http://www.alanwood.net/unicode/latin_extended_additional.html