spell check
The Thunderbird spell checking tool seems unable to manage words containing punctuation in the middle or at the end. That applies especially to abbreviations like "Dr." or "Mr." or, to give an example from the Italian language, "Dott.sa" (which refers to a female doctor). After adding such words to the dictionary, the tool still won't recognize them. Now I could point out that according to the grammar rules (of many languages other than English in addition to English itself) the punctuation inside or at the end of an abbreviation is part of the abbreviation itself, that is, the words are "Mr.", "Dr.", etc. not "Mr", "Dr", etc, but that's not the point: there is no reason for the spell checking tool not recognizing words explicitly added to its dictionary, no matter what kind of characters they contain. The meaning of a spell checking tool is very simple: if the word exists in the dictionary, then it must recognize it. If I'm missing some settings please tell me so, otherwise the tool is not working properly.
Alle svar (3)
What is your version of TB and which dictionary, name and version, are you using for spellcheck?
Software version is current. I have got an additional "Italian dictionary based upon version 5.0.0 (2016-02-10) of the Italian dictionary available for OpenOffice", but this makes no difference since the issue is related to the spell checking tool, not to the dictionaries.
The spellchecking is wholly determined by the dictionary. In this case, the Italian dictionary you have does seem to flag abbreviations such as Sig. and Sig.ra, but not Dr. By comparison, my English dictionary doesn't flag Mr. or Mrs. You can try to add items to the personal dictionary, but if they aren't recognized, it indicates a problem with the dictionary, which I urge you to bring to the attention of the dictionary add-on's author.