Realistically, the corporation makes so much more money from Firefox search deals versus foundation donations that this shouldn't be necessary. But as long as the Mozilla leadership wants to screw Firefox they will find a way. For example, by removing an equal number of engineers from Firefox development on the corporation side as the foundation hires.
They have been unable to find alternative revenue streams.
TBH it is hard to make a recurring revenue of $ 400M even from scratch , Mozilla has an existing culture that is not simply geared towards this kind of product building . I would be skeptical of even competent board and management doing this .
True but they also didn't need to had they made decisions geared around long term sustainability. They could have kept Firefox running with a budget of under $150 million a year (~500 employees) and invested the rest. That'd be a nest egg of probably $4 billion by now including growth. That's enough for them to keep going for decades with no external revenue.
Instead they kept spending money on moonshot projects to become an independent megacorp. Failing every time. Now they don't have many options remaining.
The corporation basically exists to make the Google Search Deal possible. And considering that the search deal pays more than donations to the Apache Foundation and the Mozilla Foundation combined, it looks like it was a smart move.
* Total yearly income for the Apache foundation according to https://www.apache.org/foundation/docs/FY2020AnnualReport.pd... was 2.2M
* Total yearly income for the Mozilla Foundation according to https://assets.mozilla.net/annualreport/2018/mozilla-2018-fo... was $27M, about half from "program service revenue" (which means it came from the Corporation).
* According to https://www.mozilla.org/en-US/foundation/annualreport/2018/ (different document, but notice that both are for year 2018), the Mozilla Corporation got $435M from royalties, subscriptions, and advertising. In other words, while the Foundation is not technically allowed to give the Corporation money, that doesn't seem to be a problem.
I wouldn't ask why the need for the corporation. I'd ask why the need for the foundation. Though I'm pretty sure I know the answer: the foundation exists to limit the exploitative behaviour of the corporation. Remember: Benefit Corporations didn't exist at the time.
To quote their original documentation at https://www-archive.mozilla.org/reorganization/#q2
> By forming a commercial subsidiary, the revenue-generating activities of the new entity can provide funds to support development, testing, and productization of the various Mozilla open source technologies. This benefits both end-users of Firefox and Thunderbird, and developers and others who want to use the Mozilla open source code in various ways. Having the Mozilla Corporation handle revenue-generating activities associated with these products also allows the Mozilla Foundation to achieve its goals while still itself remaining a tax-exempt organization.