Adding "From Addresses" (rather than just the forced default address)

I use Thunderbird Nebula to access my Protonmail account. In Protonmail, I have over 20 aliases for my email address. (Each address comes into my main mail box.) When I use my Protonmail account on a webpage, I can change the "From address" to be any of my 20 aliases when I reply to or forward an email. However, when I use Thunderbird to access my Protonmail, it only allows me to forward email from my default address, not my aliases. This is a deal breaker for me because '''I must be able to send email from any of my aliases'''. When I try to forward from a different custom from address, I get a message that it is possible, to send from different addresses, but I cannot find any directions online or in forums or in help or in the menus. Can anyone help me, please? Customize From Address