It's just something I've done for years without even really knowing the technical aspect of mail protocols, methods etc.
[I just "have it in" for scum, wholesale.
]As such, I consistently forward mails to the same address, which is saved and pops up as a suggestion the moment I begin to type in the "To:" field.
In Thunderbird, pressing Enter/Return (thinking that it will confirm the addressee and send the mail) will move to an additional field in order to add another recipient.
In Thunderbird, the keyboard shortcut to send the mail is Ctrl+Enter/Return, so, as soon as the correct recipient is highlighted, Ctrl+Enter both confirms the single recipient and proceeds to send the mail.
Now!...
In Fossamail, the same shortcuts work but it seems that, once the recipient is highlighted and Ctrl+Enter are pressed (Ctrl being held before Enter is tapped) Enter is being registered without Ctrl so that the additional recipient field is activated.
Anyway, I've only noticed this "quirk" due to my own methods employed in order to forward such spam on - requiring the least amount of keyboard/mouse interaction from me.