I would instead argue that Firefox's Gecko engine is beyond saving and that any money invested in it now would be better donated to other community projects because there isn't enough resource to catch up. Sticking with Gecko will eventually lead to the dismise of Firefox the organization.
Microsoft, with their resource and their ability to bundle Microsoft Edge in with Windows, couldn't get any appreciable amount of marketshare. Firefox, with less resource than Microsoft, won't fare any better.
Rebuilding Firefox with Chronium would salvage whatever the mindshare/marketshare left. Then Firefox could still wield some influence with their marketshare and the threat of forking Chronium.
Microsoft, with their resource and their ability to bundle Microsoft Edge in with Windows, couldn't get any appreciable amount of marketshare. Firefox, with less resource than Microsoft, won't fare any better.
Rebuilding Firefox with Chronium would salvage whatever the mindshare/marketshare left. Then Firefox could still wield some influence with their marketshare and the threat of forking Chronium.