The options open to you can be found in several places. When writing a message, look under '''Options '''in the Write window's menu. No menu there? Press F10 or alt+v, then go to '''View|Toolbars''' and tick the checkboxes. There are also per-account settings under '''Account Settings|Return Receipts''', and global settings under '''Tools|Options|General→Return Receipts…''' Read Receipts lead to a dialogue or pop-up that invites the reader to acknowledge having read your message. This of course is entirely optional for your reader. DSN is not widely supported. It will tell you, when it can, if a message has been delivered to the recipient's computer. It can't say if the message has been either opened or read.

Oh, and there is this: http://kb.mozillazine.org/Figuring_out_whether_the_recipient_read_your_message