In the last few days I have received a number of complaints that a Firefox update available in a PPA is broken. This is being tracked by bug #1850529.
Said PPA is ppa:ubuntu-mozilla-security/ppa and its description reads:
Staging PPA for Mozilla and other browser-related security updates. Unless you are testing updates, you should NOT install packages from this PPA.
It is used by the Firefox and Thunderbird Ubuntu packagers to prepare and test updates, it is not meant for consumption by end users.
The problematic build is being worked on (refer to the bug report for details).
I strongly suggest everyone remove that PPA from their apt sources:
sudo add-apt-repository -r -y ppa:ubuntu-mozilla-security/ppa
and wait patiently for the update to be available in the archive.