Both of them! :(

Where are you trying to type, and what happens when you do try to type in a right-to-left script? I've typed some notes in Arabic script without any problems, so I know at least something works.

A right-to-left script is a writing system like the Arabic or Hebrew alphabets; if you are writing in Persian, then you are using one already. I have sometimes seen the cursor move in unexpected ways when editing a mixture of old Turkic and English (Arabic and Latin scripts), but I can't replicate the behavior now. I am using the newest development builds of Zotero, so maybe this was something that was fixed recently.

To make things completely clear, try typing a short text with mixed Persian and English in your word processor. Then try typing the same text in a Zotero note. If the behavior in Zotero differs from that in the word processor, explain what precisely occurs, and what you expect to happen.

The handling of mixed left-to-right and right-to-left scripts is a little messy no matter what, so it will never be a completely clean switch; in particular, deleting and arrow keys can be disorienting. The key question is whether Zotero is violating the standards set by other programs.

One issue is that if your primary locale is left-to-right (i.e., Firefox English), then I don't think it is possible to type mixed text where the primary text direction is right-to-left (and vice-versa, I think). This means that when you want to type a paragraph in Persian with two or three English words interspersed, then the English will show up to the right of the entire Persian text, rather than to the left of it.
For example:

I read عبدیاحمان سهعدی the other day.

This is fine and not a problem. Embedding English in Turkic is not going to work, however:

عبدیاحمان سهعدی (Abdurrahman Sa'adi) کیم؟

In this example, the section "عبدیاحمان سهعدی" should be at the right, followed by "(Abdurrahman Sa'adi)", then "کیم؟".

This problem is not a Zotero bug, but rather isn't handled in Firefox. I don't know how to handle this cleanly-- if you know of any software that does it satisfactorily, please explain how it does any better.

for example I can write in this richtextbox -on firefox- by right click and tick "Change text direction".

Is this enabled by some sort of extension? I don't get such an option (Firefox 3.6). This would be great to have, of course. Can you confirm that Zotero doesn't handle this correctly even in RTL locales (i.e., Persian-localized Firefox & Zotero)?

Please do let us know how it goes.

Great work. Thanks for documenting the procedure in such detail.

Devs: I agree that this would be good to add to the standard installation of Zotero. RTL/LTR isn't needed by everyone, but it's part of the standard TinyMCE installation and important for a significant portion of the user base. I think we can spare the space for two buttons.