Actually Firefox or other browsers are just front ends that helps you to view the emails. All the work and the settings are at the webmail provider's end. The only thing you can do is to check if there is a delay period setting for the sending and then canceling.

Most of the time email is like the spoken word. The only exceptions are some enterprise mail applications with the option to call back an unread mail.

I know that option is available at my work through Outlook.

Hi,

I think gmail has an option to delay sending by around 30 secs. So it may be possible that other webmail providers have a similar option and maybe with a bigger delay period.