/etc/apt/sources.list.d/ add bionic sources, so it will be easier to remove it. To do this, create the file
/etc/apt/sources.list.d/bionic.list with the following contents.
deb http://archive.ubuntu.com/ubuntu/ bionic main restricted universe multiverse
deb http://security.ubuntu.com/ubuntu/ bionic-security main restricted universe multiverse
deb http://archive.ubuntu.com/ubuntu/ bionic-updates main restricted universe multiverse
#preference for firefox from bionic
Explanation: Allow installing firefox from bionic
Pin: release a=bionic-updates
Explanation: Avoid other packages from the bionic repo.
Pin: release a=bionic
sudo apt update
first simulation, because I don't no if some ppa's included in your system.
apt -s full-upgrade
NOTE: This is only a simulation!
apt needs root privileges for real execution.
Keep also in mind that locking is deactivated,
so don't depend on the relevance to the real current situation!
Reading package lists... Done
Building dependency tree
Reading state information... Done
Calculating upgrade... Done
The following packages will be DOWNGRADED:
0 upgraded, 0 newly installed, 1 downgraded, 0 to remove and 0 not upgraded.
Inst firefox [84.0+build3-0ubuntu0.20.04.1] (84.0+build3-0ubuntu0.18.04.1 Ubuntu:18.04/bionic [amd64])
Conf firefox (84.0+build3-0ubuntu0.18.04.1 Ubuntu:18.04/bionic [amd64])
sudo apt full-upgrade
to install firefox from Ubuntu 18.04.