The most money are paid to the same managers in both cases. The total company expenses in the same period mostly aren't dependent on the nature of the changes implemented. The managers just make their managerial decisions what to set as the goals.

And apparently the board thumbs that up. That's the scope of the problem: "look we make the UI changes" is the "color of the shed" easy to understand illusion of "something" being done.

Money is fungible, employees are not. Whether others at Mozilla may be tasked with implementing this API or Mozilla (or someone else) contracts Igalia to do it, the employees responsible for and qualified to work on UI are still going to get paid and will still have other work to do.