They do, see /etc/firefox-esr/firefox-esr.js -- but the aforementioned settings are not in that file by default, and [0] seems to suggest Debian does not alter the compiled-in defaults either.

Some quick digging in the source suggests that it's simply not enabled by default in ESR 128. I don't know if that's because it's only enabled by default in a later release, or because it's disabled in all ESR releases; I suspect the former. Compare [1] and [2]:

  -pref("browser.ml.enable", false); # in upstream/128.14.0esr
  +pref("browser.ml.enable", true); # in upstream/142.0.1

The other pref, browser.ml.chat.enable[d] is not mentioned in that file at all.

(edit: according to [3a] and [3b], it's browser.ml.enable and browser.ml.chat.enabled... yay for consistency, I guess)

[0] https://sources.debian.org/src/firefox-esr/128.14.0esr-1~deb...

[1] https://salsa.debian.org/mozilla-team/firefox/-/blame/upstre...

[2] https://salsa.debian.org/mozilla-team/firefox/-/blame/upstre...

[3a] https://salsa.debian.org/mozilla-team/firefox/-/blame/esr128...

[3b] https://salsa.debian.org/mozilla-team/firefox/-/blame/esr128...

▲ marc_abonce 6 days ago | parent [-]

Thanks for the heads-up! Yeah, I'm running ESR 128 right now so when I upgrade to the next ESR I'll keep an eye on these settings.

	▲	styanax 5 days ago \| parent [-]
		You can preload them now in your profile `user.js` - FF will ignore any settings it does not know about, it's "safe" to leave old things that got deleted and add new things coming in the next ESR without harm (that I'm aware, been doing it for years). A user.js is portable, not relying on any given vendor configurations.