Here is another user that complains about this issue on Github: Cannot move camera if mouse button down while pointer locked in Firefox · Issue #7797 · BabylonJS/Babylon.js · GitHub. There is another babylon link that demos the same issue in that link.
Here is another user: Firefox pointerlock + mousemove + mousedown behavior - Questions - Babylon.js
They posted a potential solution but I couldn’t get it working.
Seems like for most people the issue is broken.