I assume they're talking about the use of google analytics on the about:addons page discussed here: https://github.com/mozilla/addons-frontend/issues/2785

Two things to note from that thread:

From what I've read, Mozilla had some agreement with Google to keep all the data from their tracking stuff silo'ed so it wasn't aggregated with other data and wasn't used for anything besides showing back to Mozilla people.

and

They're now (and have been for more than a year) respecting DNT, removing the GA code so that you don't make any requests to google if you have that enabled.

So is it disabled by default since Firefox now sends Do Not Track by default?

No. It's excluded by default, and only blocked if the user manually enables Do Not Track in the Preferences section.