The answer changes every day. ;-)

Firefox, regular release, is version 88.0.1.

Firefox Developer Edition is a specially themed version of Firefox Beta, meaning both are future Firefox 89, currently on beta release 13 about 2/3 of the way through the beta cycle. If you want to run the regular release of Firefox alongside another version, Firefox Developer is a better choice than beta because it won't upconvert your regular Firefox profile (beta can do that).

Firefox Nightly is the alpha test version, two versions ahead of regular Firefox, so future Firefox 90. If you can't tolerate daily updates, you probably do not want this one.

Note that you can find on the HG website what checkins land for the various branches.

• https://hg.mozilla.org/

Nightly:

• https://hg.mozilla.org/mozilla-central/

See also:

• https://mrotherguy.github.io/fx-nightly-changelog/changes/2021/05/index.html
• https://blog.nightly.mozilla.org/

Other versions (Beta,DE,ESR):

• https://hg.mozilla.org/releases
• https://hg.mozilla.org/releases/mozilla-beta
• https://hg.mozilla.org/releases/mozilla-release/
• https://hg.mozilla.org/releases/mozilla-esr78/