This is a misinterpretation of what they are saying. Firefox has a "Reset Firefox" feature that will reset many of the settings within Firefox that may be causing a poor browsing experience.

Previously, when the "Reset Firefox" feature was used, all of your open tabs would get lost in the shuffle. This has now been fixed, and you won't lose your open tabs and current browsing session when Firefox restarts with a fresh and clean profile.

I believe 'session' and 'session cookies' are different things here.

IMO It's a rather poorly described change.

A Firefox reset is a maintenance operation, they don't mean restarting FF.