It isn't just "Mozilla Execs", it's specifically Mitchell Baker[1] because of:
> "In 2018 she received a total of $2,458,350 in compensation from Mozilla, which represents a 400% payrise since 2008.[14] On the same period, Firefox marketshare was down 85%. When asked about her salary she stated "I learned that my pay was about an 80% discount to market. Meaning that competitive roles elsewhere were paying about 5 times as much. That's too big a discount to ask people and their families to commit to.""
> "By 2020, her salary had risen to over $3 million. In the same year the Mozilla Corporation laid off approximately 250 employees due to shrinking revenues. Baker blamed this on the Coronavirus pandemic.[15]"
That is, the people who developed one of the more hyped new programming languages of the last decade were let go, the main MDN team is gone, FireFox OS is gone, long-standing FireFox loyalists were given reduced customisations and a move towards a copy-of-Chrome-but-worse experience, marketshare is down enormously since the peak, there's been a churn of janky also-rans like promoting Goya beans or something, who even knows.
What have other CEOs done to justify their salaries? Microsoft's share price is 7x higher since Satya Nadella took over. Tesla's share price is 15x higher since 2018. Amazon's share price has almost tripled since 2018. Apple's share price has more than tripled since 2018. Facebook almost doubled since 2018 (but has fallen some). CEO thinks it's unfair that other CEOs get paid more??
It's not even uniquely greedy, if the service is going well. I am annoyed at travel tickets for trains in the UK which are more expensive than flying, often late, all too often don't turn up at all, often crowded to the point of cramped standing room with the argument that "our contracts prevent us buying more carriages". If then the CEOs were saying "it's unfair that well run travel company CEOs earn more, so we're going to raise our salaries" that would be annoying. (They probably even do say that, but they have the decency to keep it behind closed doors, or to make up something about doing a good job).
[1] https://en.wikipedia.org/wiki/Mitchell_Baker