Yes. Now if you use apt to install Firefox or Thunderbird, it will reinstall snap and install the snap versions of those programs.
If you blacklist snap, it’ll throw an error when you try to install Firefox or Thunderbird cause it can’t resolve their “dependencies”.
You’ll have to install those programs from outside of Ubuntu’s repositories, and the list of affected programs is growing.
Ubuntu’s stated goal is to eventually use snap for all userland apps.