Bug - https://bugzilla.mozilla.org/show_bug.cgi?id=2077526. It's fixed, should land in release versions soon. As a workaround for now, you can drag and drop the file to be attached into the message.
@Caviedogz, support requests aren't allowed here. Please post your question with full details athttps://support.mozilla.org/en-US/questions/new/firefox/form.
@n4472x, support requests aren't allowed here. Please post your question with full details athttps://support.mozilla.org/en-US/questions/new/thunderbird/form.
