Add the Arabic Encodings to the menu via View | Character Encodings | Customize. I've uploaded a screenshot for you.

Then select Arabic which you'll see in the Character Encodings menu afterwards.

The problem is with the Arabic vowelization appearance the on XP systems with FF4 the vowelization takes a space of its own as if it is a character while it is supposed not to take any. Only to add shapes that describe the letter sound like french letters latex do. I've uploaded a screenshot for you.

If you uncheck the option to allow websites to use their own fonts, does it make any difference?

The setting can be found in Tools | Options | Content and then click the Advanced button in the Fonts & Colors menu.

Nothing happened the problem still exists

Do you have an Arabic version of Firefox installed which isn't displaying the vowelization correctly, or are you trying to read it using an English language version?

There are a few add-ons for Arabic which might possibly help here: Arabic add-ons

That is a bug that has been fixed already in the current Firefox nightly builds.

• Bug 635639 - Vowels are not rendered correctly in some Persian/Arabic/Hebrew fonts