Find a file called " persdict.dat" in your Profile folder. This is a list of words you have added to your personal dictionary. It's a simple text file list of words you have added, one word per line. It can be viewed and edited with any plain text editor (NOT a word processor.) Be sure to close down Thunderbird first.

Or you could use the edit button when checking the spelling of a document.

Between the two suggestions above, I was able to work out this answer: C:\Users\jmrhi\AppData\Roaming\Thunderbird\Profiles\l3s7y169.default-release\persdict.dat where jmrhi is my username.

When asked, I selected Notepad as the app in which to view that file.

Thanks!

Could somebody escalate this to the developers so a more user-friendly solution could emerge?

I don't think you could get much more user friendly than Matt's answer above. That said, you can submit and vote on product ideas and suggestions at the Ideas section of Mozilla Connect:

https://connect.mozilla.org/