Whenever I install Firefox on a new computer, I find myself hitting Ctrl-Shift-X to switch the text box direction to now avail. Unfortunately the way to turn this option on is quite hidden.
You have to navigate to about:config, type bidi in the filter box, and double click the line that says bidi.browser.ui. That will set its value to true. All you have to do now is restart Firefox and the View->Switch Page Direction and Edit->Switch Text Direction menu items will be added along with the Ctrl-Shift-X shortcut.
Hope this helps some poor soul someday.