There are currently 4 organizations on the planet that build their own browser engine: Apple, Google, Microsoft, and Mozilla. Which one of these is not like the others? That's right, Mozilla is the only browser vendor that isn't a giant tech company that already has lots of money from other sources.
Not saying this excuses installing a rando extension for the sake of a publicity stunt, but I can see why Mozilla might try to find some… creative sources of extra income.