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.
I'm 100% onboard with Chromium being the universal de facto standard for the web as long as it's open.
Really, all I want from a browser is Chromium, full features without disabled APIs, with a few extras like Sync, P2P stuff, and codecs, fully open.
Right now, I think anyone with name recognition and marketing ability could probably develop a winning browser for 50k or so. Just... take chromium, add sync, and an ad blocker for high bandwidth video ads. Done. You have made the world's best browser, the rest is business stuff.
Mozilla itself didn't think the Gecko engine itself has a long term future either. After all, Mozilla funded the development of the new Servo engine for eight whole years, and it slowly replaced pieces of Gecko with Servo.
After Mozilla laid off all Servo developers in 2020, the future doesn't look bright for Gecko or Firefox.
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.