Install Firefox Quantum in debian 9 Stretch

Add deb http://ftp.hr.debian.org/debian sid main contrib non-free to /etc/apt/sources.list and install it with this command:

apt install -t sid firefox

This will install only Firefox from unstable. Rest of packages will remain on stretch.

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Added by cas 2018-04-19 (because it's quite common for people to want to install something from unstable without upgrading everything to unstable, and the answer here is applicable to more than just firefox):

This is a good answer, but incomplete. There are two more things that need to be done before running apt install -t sid firefox.

1. Add APT::Default-Release "stable"; to /etc/apt/apt.conf or a file in /etc/apt/apt.conf.d/ so that apt will only install packages from sid/unstable if you explicitly tell it to with -t sid.

   If you don't set the default release to stable, the next upgrade or dist-upgrade will upgrade your entire system to sid. Most people don't want this.

   If you're using a named Debian distribution such as jessie or stretch in your sources.list file, use that name rather than the generic stable.

2. run apt update to update the local package database.

Finally, apt install -t sid firefox will install not only the firefox package but also the minimum set of upgraded & new packages required to satisfy the new firefox package's dependencies. This will usually just be a few firefox-related packages, built from the same source, but may also include other packages - e.g. if the new firefox depends on a newer version of a library package.

Sometimes it may even cause an important package like libc6 to be upgraded which will then trigger a huge cascade of other package upgrades, effectively upgrading you to a hybrid of stable & unstable. This is generally worse than doing a full dist-upgrade to unstable itself. If this happens, you have two good choices: 1. cancel the firefox upgrade and wait for it to arrive in stable or https://backports.debian.org/; 2. cancel it and upgrade to unstable (which is not as bad as it sounds. In Debian, "unstable" doesn't mean "will crash all the time". It means "pre-release, changes constantly. sometimes things may break and require manual fixing")

https://blog.mozilla.org/blog/2017/09/26/firefox-quantum-beta-developer-edition/

If you’re already among the Firefox faithful, you’ll automatically upgrade to Firefox Quantum on November 14. But, if you enjoy the cutting edge, you can try it in Beta on desktop, Android, and iOS. Or, if you’re a web developer, download Developer Edition, which includes brand new, cutting-edge tools for those who build the web.

you simply could wait a few weeks. or you could download the beta which is in tar.gz format

so seems Quantum is not an "all new firefox" but an update.

hey, if it really is only about installing a beta version, then you may simply add the Firefox-beta Repo: https://launchpad.net/~mozillateam/+archive/ubuntu/firefox-next

EDIT: It's now in unstable, so the link changed to https://packages.debian.org/sid/firefox

OLD ANSWER: Firefox Quantum is available in debian experimental (https://packages.debian.org/experimental/firefox). You can probably just download and install the deb file from there.