I don’t know if there is a specific add-on for that for Firefox, but I just have “Ask before displaying external images” selected in my gmail-settings. Any external image shown in an email can potentially be used as a “read confirmation”.
It hides images that are 1pixel x 1pixel, and also images retrieved from a short blacklist of sites. I wonder whether the author is considering porting the extension to Firefox? Based on some light testing, a few changes would be required to make it run. I don’t know jQuery well enough to get very far with the project, and I’m not aware that it has an open source license, so I probably couldn’t publish a tweaked version for Firefox anyway.