de asis,
Welcome to the forum.
The Arabic language is one of the most beautiful languages of the world. I remember have read some texts where they explained the structure of the Arabic, which is similar to Spanish, in the sense that you can say one thing in multiple ways, with different words and with no confusing meanings at all. It’s just amazing. Arabic is a rich language. Wish I could read Al Jazeera’s site in Arabic. Well, I can read Al Jazeera English at least. I should not be complainin’ at all. 
OK, that’s enough offtopic jive. :D:D
I don’t know if CometNetworks will create an ar locale for CometBird, but today I have a temporary workaround for you: Arabic language pack!
If you wanna have CometBird in Arabic, just follow these easy steps:
■ 1. First, click here. By clicking here, you’ll be prompted to install the XPI file into CometBird with all the Arabic text strings. You should let CometBird install this.
Once is fully downloaded, you’ll see a new item in the Add-on window: Languages. CometBird needs to be restarted, but don’t do it yet.

Let’s go to the second step. Leave the Add-on window opened, don’t close it.
■ 2. In the web browser’s main window, at the address bar, type about:config and hit the Enter key. You may see a warning, click on the I’ll be careful, I promise! button.

Now you’ll see a really long list of crazy things and the cursor blinking in the Filter form. In this form, type general.useragent.locale. That’s the name of the preference we need to modify. OK, you’ll see now en-US under the Value column. Double click on en-US. The Enter string value window will pop up. Clear the default en-US value and enter ar.

Now click on the OK button. The general.useragent.locale line had became bold, this means that the preference has been modified from its original state. Now, back to the Add-on window and click on the Restart CometBird button. All CometBird’s windows will be closed, of course.
A few seconds later, CometBird will be running again, but now in Arabic. B)
Now, please, let me add three things that may enhance your CometBird experience.
■ This is an optional step, it’s not mandatory. In order to have always the latest Arabic language pack installed, you have to allow CometBird to check automatically if there’s a new version of this language pack. I repeat: this’s optional, not mandatory. If you want to do this, just keep on reading the next paragraph.
Run CometBird and go to Tools > Options… > Security. Now if you have checked the Warn me when sites try to install add-ons box (you should had it checked, for security reasons), click on the Exceptions… button, to the right. The Allowed Sites - Add-ons Installation window will appear. At the Address of web site form, enter: releases.mozilla.org and click on the Allow button. Then, close this window and press the OK button at the previously opened Options window.
■ The steps 1 and 2 above are about the web browser interface language, if you want to customize the web sites/browsing languages, run CometBird and go to Tools > Options… > Content > Languages section > and click on the Choose… button next to Choose your preferred language for displaying pages. At the Language window you’ll be able to select one or more languages of your preference.
■ Have in mind that after installing the language pack, the web browser will call itself Mozilla Firefox rather than CometBird, because this language pack is designed for use it with Firefox, but this works just fine with CometBird. Also, some text strings (sentences, descriptions, warnings, options, etc) will keep in their original en-US locale, like those strings from some add-ons.
Best regards,