Rated 5 out of 5 stars

After being overwhelmed by the number of replies I decided to have a go myself, the following "appears" to be a fix!!!!
To get the British English directory to work:
Extract the .xpi file with winrar or winzip etc,
open install.rdf and change all the options of "max version"
to what you are using, [at the moment 12.0.1 for me] and save the file.
Put it back into the archive with win rar etc [copy and replace].
Install dictionary as an add-on by "installing from file" useing the
new .xpi and it seems to install and stay after reboot ok.
Seems to work on the normal and portable version across several pc's!
If you put in say version 20 as the max you might future proof it?
I might try that soon when I'm feeling keen, have a go or get a
friendly geek to do it for you, it's not too difficult.
Which leads me to the question "Why on earth does the owner David Bartlett not do this????".

This review is for a previous version of the add-on (1.19.1).  This user has a previous review of this add-on.