Be careful with old versions!
These versions are displayed for reference and testing purposes. You should always use the latest version of an add-on.
1) When switching to 'Plain Text' and then back to 'Rich Formatting' the RTL and LTR buttons do not reappear.
2) Also if you click the 'Compose Mail' link again when you are already in the compose mail screen the RTL and LTR buttons disappear.