Rated 5 out of 5 stars

Excellent once again!

This rather strange problem was resolved by the simple expedient of setting QLS preference
"Reload the current ..... after switching" to No.
My thanks to author Martijn (Captain Caveman) for a prompt response.
I leave a partial record of the problem for archive:

Has been excellent.
However, a warning for those moving to FF 15.0 : I have experienced some truly bizarre behaviour, where pages (like gmail.com & yandex.ru & even the AMO) keep cyclically re-rendering & re-rendering, sometimes blank, sometimes without CSS, and won't be stopped.
== SOLUTION: if the AMO (Add-ons Mozilla) page quiets down enough,
== change the QLS preference "Reload the current ..... after switching" to No.

All of which seemed to be related to QLS, so I removed it, which left all the main FF interfaces speaking Russian at me.
== The change to Русский язык remains mystery, but may well not be related to QLS.
== The solution is to go FFox Edit> Prefs > Content > Languages > Choose >
== highlight your preferred lingo & [Move up] to top language.

This review is for a previous version of the add-on (