With the CEO getting that much, they do have to be careful about other costs.

Technically Google is the one paying Mozilla CEO's $7m salary.

Clearly a good investment, Firefox market share is lower and lower, and has been steadily declining since Google became Mozilla's main income source.

why 2 M in $7MM?

2 millions millions dollars? that's 10^12 dollars.

It's the accepted financial notation, perhaps you're confusing it with 7M$ which would be an SI like encoding.

https://corporatefinanceinstitute.com/resources/fixed-income...

That page says MM is antiquated, falling out of favor, and M is the more modern way to represent a million.

You have to be careful about separating your interpretation of what something is saying when referring to the claims of the content directly like that. Nowhere does the page say antiquated, that's just one particularly strong interpretation of "becoming less common". It'll probably eventually be antiquated but it certainly isn't yet - it's still very popular and accepted.

Because finance has weird conventions about numbers, and $7M could be misread as $7000 in the wrong context

Roman numerals. 1000 1000

Then it would be 2000