Review for British English Dictionary by Marco A.G.Pinto
Rated 4 out of 5 stars
@Stewart
This add-on hasn't been maintained for several years.
The official one is now mine: "British English Dictionary (Marco Pinto)".
Please notice that Mozilla updated all Firefox spellers to WebExtensions, but they only work with Firefox 62+ due to a bug in Firefox 60ESR.
Thunderbird still uses Legacy Extensions, so you can download the version here in ATN.
To create your own collections, you must have a Mozilla Add-ons account.