Oh wow. So from a financial standpoint they're not in a radically different position to earlier years (assuming between 10-20% more in other revenues), in fact seems as if they have approx 100M$ more coming in as revenue in comparison to the previous year of comparison (2018) [1]

[1] https://en.wikipedia.org/wiki/Mozilla_Corporation#Google

Then why in the name of space balls did they fire like 25% of their dev team?

Because they they already re-upped it, why spend more money when you already got the rewards? That is what happens when a company doesn't care about the product.

Because income is lower than expenses.