That is the cost of these employees to their employer ("fully loaded"). The fully loaded cost is typically about twice the employee's nominal compensation, which would be ~$113k. This is about 60% of entry level comp at a FAANG company and about 35% of mid-career comp. [1]
Mozilla employs a lot of people who are willing to work under market rate to do a job they believe in, but I doubt they would be able to get and retain the people they need if they offered substantially less.
[1] https://www.levels.fyi/?compare=Google,Facebook,Apple,Amazon...