I won't say no, but these donations go to Mozilla Foundation and are used to advance their tech evangelism work, such as their successful lawsuit over net neutrality in the US.

Mozilla Corporation develops Firefox with their income from sources like their contract with Google and other search engines. Donations are not used for Firefox dev.

So what software is the Mozilla Foundation developing that's costing them over $200 million a year that isn't Firefox?

Sorry, what? Mozilla Foundation doesn't do software development.

Their financial statement says they do.

https://www.mozilla.org/en-US/foundation/annualreport/2017/

The way it's worded it sounds like they do it through the Mozilla Corporation, which they own wholly own.