Go to the Mozilla Add-ons Web Page {web link} (There’s a lot of good stuff here) and search for what you want.

Gmail; https://addons.mozilla.org/en-US/firefox/search/?q=Gmail

I did that BEFORE I asked the question. Not helpful.

Please explain.

I want to get a receipt or notice when someone opens an email I sent.

Ahh, that is a provider issue. Not Firefox. Contact the e-mail support, and ask about "return receipts."

Are you using MailTrack? Interesting product.

The MailTrack extension/service embeds a tracking image, sometimes called a web bug, into your outgoing Gmail messages. They also modify links in your message to redirect through their servers. By default, they promote MailTrack in your email signature if you have a free account.

When a recipient loads the message, if they also load the images, MailTrack records that information. In fact, they record not just the first load, but "all the days and times they have been read, the device used (PC, smartphone, etc.)." That includes not just an individual message but a conversation containing the message (if the reader uses a conversation view). "MailTrack is also able to give the same information regarding the links you included in your message."

In order to embed that information back into your mailbox, you give MailTrack some level of access to your Gmail account -- I don't really understand how that part works. They say:

This is the only information that we access when you use MailTrack: the sender and recipient addresses, the links included in your message and the subject of your email. It is only the necessary information we need for tracking and properly classifying those emails for you.

While I'm not aware of anything like that for Firefox that works on the normal Gmail site, it wouldn't surprise me if there was something from one of the major bulk mailing companies that you would use through their site.

Hmm, apparently there are others that didn't show up when I tried the Chrome store... http://www.howtodigitalstuff.com/gmail-tracking-software/