anecdotal reports are that this makes Firefox noticeably faster

that list of settings to disable:

browser.ml.enable
browser.ml.chat.enabled
browser.ml.chat.menu
browser.ml.chat.page
browser.ml.chat.page.footerBadge
browser.ml.chat.page.menuBadge
browser.ml.linkPreview.enabled
browser.ml.pageAssist.enabled
browser.tabs.groups.smart.enabled
browser.tabs.groups.smart.userEnabled
extensions.ml.enabled
browser.search.visualSearch.featureGate