I just uploaded a new version of the Force RTL extension, which finally fixes the extension to work on recent Firefox and Thunderbird versions (Firefox 3.6 and Thunderbird 3 betas and above). This new version of the extension adds support for the new intl.uidirection.ab-CD preference added in bug 478416 by Neil Deakin.
So, if you are a Firefox or Thunderbird developer and want to know how the application looks like in RTL locales (Arabic, Hebrew and Persian), or if you’re simply curious about it, and don’t want to learning one of those languages so that you can actually use a localized build, please go and download version 2.1 of Force RTL. To toggle between LTR and RTL mode, simply click the Force RTL Direction menu item in the Tools menu. No restarts, no hassles!
As always, feedback is much appreciated.