Sounds that Firefox uses a font on that site that doesn't render Persian fonts properly.

Try to disable Direct2D by setting the gfx.direct2d.disabled pref to true on the about:config page.

Try to disable hardware acceleration.

• Tools > Options > Advanced > General > Browsing: "Use hardware acceleration when available"

Try to disable the website fonts as a test and try a few different default (serif/sans-serif) fonts.

• Tools > Options > Content : Fonts & Colors > Advanced ([ ] Allow pages to choose their own fonts, instead of my selections above)

I tried eht second option...it works, the fonts on fb can be read correctly but now each website I visit has change to times new roman... but it works, thanks for your help