'Quantum' is a brand of Firefox over v57.

Sensuelas said

The more I search on Mozilla and Firefox websites, the more confused I get. Which Firefox do I need to use? I have downloaded Firefox 58.0, Firefox Quantum, Firefox Nightly. Then I also found Firefox Beta, Seamonkey and Waterfox. Any tips on why I should one or the other? I must have used Firefox browser before, because I recovered an old account. But I already created a new Firefox account, so now I have 2. Is it useful to have multiple Firefox accounts? They are merged so I wanted to delete the old account. But I thought I'd ask first to be sure I don't do anything wrong. My final question is about the difference between a Firefox account and a Mozilla account. Why do I need both if Firefox is a Mozilla product?

You do not need a Firefox Account in order to use the Firefox web browser. The main reason people create one is to sync some settings between Firefox on two devices. https://support.mozilla.org/en-US/kb/access-mozilla-services-firefox-accounts

Firefox 58.0 is currently still on the Beta development channel with current build being 58.0b13. The Firefox 58.0 Release is scheduled for January 13, 2018 if all goes well.

Quantum is referring to Firefox 57.0 and later Releases.

Nightly is a more bleeding edge development channel meant more for Testers and developers. It gets two updates with checkins each day. Sometimes this can result in issue(s) until fixed, finished, or reverted.

Release < Beta (b#) < Nightly (a1)

There is also Firefox developers Edition (Aurora) that is same as Beta but with additional developers features.

Waterfox is a third-party build not affliated with Mozilla that originally started by making 64-bit Releases of Firefox even though the 64-bit code was not considered stable until Firefox 42.0 Release. Waterfox became less useful since then.

SeaMonkey is not Firefox but rather a community effort in continuing the old Mozilla application suite after it was discontinued by Mozilla at version 1.7.13 back in April 2006.

If by Mozilla account you mean for support.mozilla.org then it is only for this site.

There do seem to be a lot of products under the "Firefox" brand.

The standard, stable release of Firefox for Windows, Mac, and Linux is Firefox version 57, also known as Firefox Quantum.

See: https://www.mozilla.org/firefox/

Other available releases for Windows, Mac, and Linux:

• Extended Support Release of Firefox 52 ("ESR"). See: https://www.mozilla.org/firefox/organizations/faq/

• Test versions: Beta (58), Developer (58), Nightly (59). See: https://www.mozilla.org/firefox/channel/desktop/

I think we can ignore Android, iOS, and FireTV for purposes of this thread.

The things that are not called Firefox, are not Firefox, although they may use some of the code of Firefox in their own ways.