The current bundle will extract the upstream binary to your home directory when run, only if it doesn’t exist already. This will always want to update and has the permissions to do so. The firefox bundle will not update this version and relies on the auto updates.
The main reason it isn’t normal to receive auto-updates, is that it’s installed to /usr. There’s two issues there, 1. no permissions for normal user to update and 2. changing installed files of the package manager is breaking behaviour.
You could remove the firefox bundle once it’s installed to the home directory. However, you would be missing 2 important files,
/usr/bin/firefox to run it from the terminal via
$PATH and a desktop entry to load it from app menus. So you’ll have to create those if you remove the bundle.
You will also miss out were firefox were to become compiled, optimized and installed via
swupd unless you reinstalled the bundle again.