I'm using Windows OS and Thunderbird 68.4.2 English (British) version and I've installed a Dictionary - the British English Dictionary (Marco Pinto) 2.81.

• https://addons.thunderbird.net/en-US/thunderbird/addon/british-english-dictionary-2/

In 'Menu app icon' > 'Options' > 'Composition' > 'Spelling' tab I have selected the checkbox 'Enable spell check as you type' and under 'Language' I've selected the 'English (United Kingdom)' option.

In a new Write message window, I typed 'eth' and it was underlined with a red squiggly line indicating a spelling mistake.

I clicked on 'Spelling' and 'the' was offered as alternate in the list, so I could select it. But make sure the 'Language' selection at the bottom of the window is the language you wan to use.

When you click on 'Spelling', you can also add your own words to the 'Personal Dictionary'. Click on 'Edit' button to add your own words. Enter the 'New Word' and click on 'Add'. The newly entered word will appear in the 'words in my dictionary' list. click on 'Close'. I got fed up with my name being flagged as incorrect and a few other words like 'can't' not being recognised, so I added them to my 'Personal Dictionary'.

I do not know what OS you use nor what version of Thunderbird nor what language eg: English British or English US or English Australian etc. You may have had a Thunderbird update and need to check whether the Dictionary is up to date as well. Check your installed dictionary is the correct one for your version of Thunderbird.

Check the 'Options > 'Composition' > 'Spelling' tab I mentioned. Have you selected the checkbox 'Enable spell check as you type' ? Is the selected language correct?

Thank you, Toad-Hall, for the response. To expand my original question and to address your response.

Linux/Fedora29 Thunderbird 68.2.2 No "Option" on the Linux version, under "Preferences" instead. Dictionary: en-us 7.68.2.2.20191105113228 'Enable spell check as you type' is selected Thunderbird loaded from usual Fedora repositories.

Adding a new word to the personal dictionary is not my problem: I understand that and have used it many times. I have even used the "edit" function to fix a mistake when added a new word that should not have been there.

To restate my problem: how to I add an alternate possible correction when spell check detects an error, such as "eth"?

I notice that with the UK, AU, US and CA English dictionaries, right-clicking on eth does not suggest the as an alternative (see picture), but clicking on the Spelling button does in fact display the, along with many more.

I don't think users are able to add items to the context-menu suggestions list, but they will probably find the correct suggestion in the extended list. The shortened context menu is probably based on the frequency of the most-intended replacement, i.e. when eth is typed, most users intend to type et, eh, etch...

Yes, sfhowes, that is so, thank you for the observations.

This then begs the question:

How to change the context menu to contain different selection from the full list, or perhaps to make the context menu longer? Certainly in my case, I need "the" instead of "Seth" or Beth"! :-)

I gather from this bug that the suggestions algorithm is controlled by the dictionary authors, not TB or Mozilla. There's an even more significant problem that words added to the Personal Dictionary don't appear in the suggestions for misspelled words.

Oh, my, ..... from 11 years ago! :-) I would expect a problem now would be ignored for another 11 years!

I wonder, however, if the context menu is a TB or Mozilla responsibility. Seems that the choice to show five alternates on that menu instead of 3 or 7 or 10 or ... would rest there and not with dictionary authors. In my simple case, 7 alternates would make me happy! :-)

Interestingly, this only became a problem for me recently. It USED to work for me to the show "the" for "eth", but disappeared. Perhaps a code update somewhere?

… There's an even more significant problem that words added to the Personal Dictionary don't appear in the suggestions for misspelled words.

Yes this is a significant problem. This was reported as a bug 14 years ago: https://bugzilla.mozilla.org/show_bug.cgi?id=338291. As I said when I added a comment to the bug report 8 months ago: This is really annoying as it is one of the few areas where Thunderbird lacks a feature which is in MS Outlook. Fixing this would make a big difference to users in Australia and other places where there are many words, especially place names and abbreviations, which are not in the standard UK English dictionary or in the local dictionary, and so have to be added to the Personal Dictionary to avoid being flagged as misspellings, but then are not offered as suggestions for typos. Perhaps now with the advances in the applications in the last 14 years, someone could have another look at this?