As far as I know, this is not possible. You can explore the many Firefox addons and see if one of those can change this feature.

If you want to leave feedback for Firefox developers, you can go to the Firefox Help menu and select Submit Feedback... or use this link. Your feedback gets collected at http://input.mozilla.org/, where a team of people read it and gather data about the most common issues.

You can see the speed in the tooltip if you hold your mouse pointer over the progress bar (see attached screenshot), but I don't think there is any convenient way to show it on the downloads panel itself.

Speaking of add-ons, this one came up in a search: https://addons.mozilla.org/firefox/addon/download-status-bar/

I think this will do what you want,

Download Status Bar {web link} If you are the kind of person who likes to have total control over every aspect of your downloads, "Download Status Bar" is the add-on you are looking for.