How can I send from a disposable email address?

I use Abine to protect my identity, and I often create disposable email addresses for websites to contact me. Occasionally I want to send an email to a company or individual but I don't want them to have my real email address (think Craigslist). But when I write an email, I'm only able to send From the "registered" email accounts in TB. Is there a way to change the "From:" to a disposable address? I have tried numerous times to create an account using a disposable address but I'm never able to succeed. Interestingly, I used to be able to do this many many TB versions ago, I don't know which one, but now I can't. Can anyone tell me how to accomplish this? This is an important privacy concern and I'm surprised Mozilla hasn't considered this. Thanks in advance for any help.