Bugzilla

User Agent: Mozilla/5.0 (X11; Fedora; Linux x86_64; rv:80.0) Gecko/20100101 Firefox/80.0

Steps to reproduce:

Fedora 32, x64
thunderbird-68.11.0-1.fc32.x86_64 

One of my project was to remove the work "yo" fro my standard dictionary (not my custom dictionary, where it does exist).  No one seems to know where Thunderbird gets it standard en_US dictionary from.  Hint: is it not from /usr/share/myspell/en_US.dic

In the process of figuring out a workaround, I came up with this:
    1) close Thunderbird
    2) as root, go to
           /usr/share/myspell
    3)  # cp en_US.aff en_DE.aff
         # cp en_US.dic en_DE.dic
    4) restart Thunderbird
        --> preferences
          --> Composition (left column)
            --> Spelling (tab upper right)
              --> Language (down arrow, midway down) 
                    Select (English Germany)

Now if you are still with me, here is THE BUG: prefs.js does not show me using en_DE.   Nothing changed before or after the workaround

By the way, if you change to the myspell version of en_US, it still searched the web:
user_pref("dictionarysearch.url1", "/usr/share/myspell/en_US.dic $");

Here is dictionary from prefs.js:
user_pref("dictionarysearch.menutext1", "Dictionary Search for \"$\"");
user_pref("dictionarysearch.menutext2", "");
user_pref("dictionarysearch.menutext3", "");
user_pref("dictionarysearch.menutext4", "");
user_pref("dictionarysearch.url1", "http://dictionary.reference.com/search?q=$"); 
user_pref("dictionarysearch.url2", "");
user_pref("dictionarysearch.url3", "");
user_pref("dictionarysearch.url4", "");

1) please clean up prefs.js for the proper dictionary entries.  Get rid of stale entries.
2) show the proper dictionary in prefs.js that also shows in preferences

User Agent: Mozilla/5.0 (X11; Fedora; Linux x86_64; rv:80.0) Gecko/20100101 Firefox/80.0

Steps to reproduce:

Fedora 32, x64
thunderbird-68.11.0-1.fc32.x86_64 

One of my project was to remove the word "yo" fro my standard dictionary (not my custom dictionary, where it does exist).  No one seems to know where Thunderbird gets it standard en_US dictionary from.  Hint: is it not from /usr/share/myspell/en_US.dic

In the process of figuring out a workaround, I came up with this:
    1) close Thunderbird
    2) as root, go to
           /usr/share/myspell
    3)  # cp en_US.aff en_DE.aff
         # cp en_US.dic en_DE.dic
    4) restart Thunderbird
        --> preferences
          --> Composition (left column)
            --> Spelling (tab upper right)
              --> Language (down arrow, midway down) 
                    Select (English Germany)

Now if you are still with me, here is THE BUG: prefs.js does not show me using en_DE.   Nothing changed before or after the workaround

By the way, if you change to the myspell version of en_US, it still searched the web:
user_pref("dictionarysearch.url1", "/usr/share/myspell/en_US.dic $");

Here is dictionary from prefs.js:
user_pref("dictionarysearch.menutext1", "Dictionary Search for \"$\"");
user_pref("dictionarysearch.menutext2", "");
user_pref("dictionarysearch.menutext3", "");
user_pref("dictionarysearch.menutext4", "");
user_pref("dictionarysearch.url1", "http://dictionary.reference.com/search?q=$"); 
user_pref("dictionarysearch.url2", "");
user_pref("dictionarysearch.url3", "");
user_pref("dictionarysearch.url4", "");

1) please clean up prefs.js for the proper dictionary entries.  Get rid of stale entries.
2) show the proper dictionary in prefs.js that also shows in preferences