Somewhat embarrassingly, I mixed up Thunderbird and Firefox. The method should work equally for both though, except that the firefox profiles are stored in
As of writing, most answers don't work straight forward or at all anymore. The closest is the recommendation to add content to
persdict.dat, but the answer of Dolanor doesn't work straight forward with modern versions of spell checkers, because the dictionary files typically don't contain all variants of the words -- but rather rules how to form them from a dictionary of base forms.
What I did instead was to use the command
to generate a list of all variants and append that to
$HOME/.thunderbird/<PROFILEFOLDER>/persdict.dat. This should work for any language; In my case I am using a German dictionary in thunderbird, with
en_US added to the
While this works in principle, there will remain cases that are not handled well due to rules that go beyond the forming of the words.
With a German main dictionary and an English generated
persdict.dat in particular, the spell checker will complain about constructs like hyphenated English words.
As pointed out by jbrock's comment, this setting also doesn't account for the possibility of English words being capitalized (e.g. title case style). They provided a solution in the comment.