A stupid way to answer the question, how many non-paid contributors were there who landed a patch in a release.
When:
- log into bugzilla.mozilla.org
- run a query, eg: number of bugs closed in WebExtension in Firefox 58 (https://mzl.la/2zA29dg)
- on the search results page click the orange page action
- review any who may or may not be paid, click on the "yes" or "no" links to change that
Wonder why I bothered doing this.