Mozilla is the primary developer of the Gecko web engine which Firefox uses. Open source or no, web engines are complicated things to develop and maintain. Even Microsoft gave up and just started using Chromium.

Would it continue to work without a significant organization dedicated to maintaining it? Sure, probably, for awhile. Would it continue to be safe to use for things like finances? No.

Is there any point to switching web browsers to one that is based on the same engine? Not really, no, the browser application is basically just a wrapper for the engine... a skin, a theme, like having an SUV shell vs a pickup shell on the same frame and drivetrain.