Nope. Debian repos (and most package managers I'd guess) do not have Nightly or Developer Edition. There's no direct link on the download page [1] that takes you to where you can download the Nightly branch. And the page for downloading it [2], to which you reach if you search for it specifically, makes it as visible as it gets that it's beta and meant for testing. I guess all of this is just lightly forcing you to download latest release. (edit: I mean this as sth. positive BTW, they're making it quite obvious which is the wiser option for the end user)
[1] https://www.mozilla.org/en-US/firefox/new/
[2] https://www.mozilla.org/en-US/firefox/channel/desktop/, "Get a sneak peek at our next generation web browser, and help us make it the best browser it can be: try Firefox Nightly."