You are not receiving update using package manager because Firefox is not yet updated to the latest version in bionic beaver on which elementary OS Juno is built. As soon as Firefox is updated in the bionic beaver, you will also be able to update it here in elementary OS.
I have a workaround not solution sadly. But I think its better to follow this instead of messing with installed source because the package will be updated shortly.
Please download the latest version of Firefox from its official website and extract it a folder. The download file mostl likely come as tar compressed file. Please open terminal in the directory where your downloaded file is stored and execute
tar -xvjf firefox-66.0.5.tar.bz2.
This will extract Firefox in "firefox" named folder. Please execute
cd firefox to change directory to that folder and execute
./firefox to launch Firefox. Make sure all instance of old version is closed before you execute last command to fire up new firefox.
New Firefox will load of everything from the installed-old one. Keep using it until its updated in the repository of Ubuntu.