Does firefox supply e-mail account as well?

Hi

No.

But you can use the Firefox Relay (The "Mask" Service) It generates "masks" (randomly generated email addresses like xyz123@mozmail.com).

You can use these masks to sign up for websites. Any mail sent to the mask gets forwarded to your real personal email. If a site starts spamming you, you just delete the mask, and your real email stays safe and clean.

No, but you can use Firefox to open email links with a webmail service such as Gmail. See Change the program used to open email links. Since you're on Windows, see also Set Firefox as your mail-to default on Windows