Bold issue in Thunderbird

Hi all, Suddently all my emails from my accounts are composed in bold. The HTML signature of each account are different, but when I compose a new email in any account, Thunderbird inserts the signature in bold. Default text is not bold, but if I change the font it switches automatically to bold and clicking bold button doesn't "unbolds" the selected text. I have tried to restore writing default values, but the issue is still there. Any clue, please? Thank you.