I'm happy to announce the availability of the Force RTL extension! This extension allows you to test Firefox and Thunderbird's user interface in RTL mode, without needing to download a Hebrew, Arabic or Persian version, without changing the user interface language, and without restarting the application (if you're running a recent version of the application). You can read more here.
Kai Liu had the original idea for this extension. He mentioned to me on IRC that he has an extension which he uses to test the browser interface in RTL mode. I was eager to see this extension, and he was kind enough to share the extension on addons.mozilla.org. I decided that it was inconvenient to restart the application each time one wants to switch the direction, so I started hacking the extension and added support for dynamic switching of the direction. After a few emails exchanged with Kai, some small changes made by him and me to make the extension work better, I uploaded the latest version of the extension (2.0.1) to addons.mozilla.org.
Go install the extension and test how your Firefox and Thunderbird look and behave in right-to-left mode! I also created a page explaining this extension in more details. Here's a picture of this extension in action: