You are talking about a 'return receipt' option. That is set up by your e-mail provider in the options.

No, I mind service like https://mailtrack.io/en/

Hello komir,

As you probably already know : Gmail doesn't have a free version of this option ....

The only add-on that might work :

https://addons.mozilla.org/en-US/firefox/addon/trockerapp/

Any good ?

Thank you for your help, but, seems to me this add-on doing contrary I need. It blocks tracking my emails, it does not show is someone opened my email in Gmail as https://mailtrack.io/en/ do? I found this add-on https://addons.mozilla.org/hr/firefox/addon/posttrack/ but last time it is updated 2 years ago and have rating 1. I install it but as expected it is not working. Why it is still on add-on list?

I am very satisfied Firefox user for many years and https://mailtrack.io/en/ was my major business extension. Till they don't support Firefox any more don't know what to do? I see Chrome have more than one extension what can do that, but, I don't like Chrome :) Is there any way to use Chrome extension on Firefox? Thanks

You're right about "Trocker" - I foolishly thought it did both (track and block) my bad, sorry ....

There is this add-on :

https://addons.mozilla.org/en-US/firefox/addon/bananatag/

But the reviews aren't very good, though.

As for using a Chrome extension with Firefox : there used to be an extension that would do that, but it was discontinued. Then I found this :

https://www.reddit.com/r/firefox/comments/dwmjf6/is_there_any_way_to_run_chrome_extensions_on/

Would that work for you ?