The original Welsh Dictionary was created by Thomas Thurman, but hasn't been updated for a long while. My intention therefore is to ensure that the Dictionary remains compatible with the latest versions of Mozilla products. This is now the "official" Welsh dictionary on Mozilla's language support page
The word-list was created as part of the Cysill software, developed by Canolfan Bedwyr, University of Wales, Bangor. I am not affiliated with this team (although they do know of my existence), and as such, maintaining the word-list in the dictionary is not my (the developer's) responsibility.
Please note that with Mozilla's new rapid-release cycle, I'm finding it hard to keep the dictionary up to date with the latest Firefox version (I'm only a part-timer and I don't get paid for this!). While I upload new versions of this software as soon as I can upon receiving a compatibility-verification e-mail from Mozilla, there is sometimes a delay while the Mozilla team have to authenticate the upgrade (to ensure I don't do anything naughty, technically speaking).
However, the good news is that I now have a Wiki "homepage" link for this add-on, where you can get the latest version. Find the link somewhere around here!
More details to be found, and discussion-style feedback can be given, there.
Finally, I'd be very grateful if you could rate the add-on here! If nobody says anything, I'll assume it's perfect!