On Ubuntu (and many other Debian-based Linux distros), automatic Firefox updates are handled through unattended-upgrades, part of the APT system. This is totally outside Firefox's control, so as you probably suspect, about:config won't help you here.
There are other resources out there on how to pin a package to a specific version:
https://askubuntu.com/questions/18654/how-to-prevent-updating-of-a-specific-package
However, this prevents updates from being installed at all, even when you manually run apt-get upgrade. Probably not what you want.
Another option is to disable automatic security updates altogether. This can be done in the "Software & Updates" application (software-properties-gtk) under the "Updates" tab. If you manually upgrade very frequently, maybe that's okay, but generally, there is no need for this, because...
You can configure unattended-upgrades to skip specific packages relatively easily:
Edit the file /etc/apt/apt.conf.d/50unattended-upgrades and find the Unattended-Upgrade::Package-Blacklist section. Add a line to block any package starting with firefox:
Unattended-Upgrade::Package-Blacklist {
// Some existing comments, maybe some other stuff
// ADD THIS LINE:
"firefox";
};
Now, APT will not update Firefox without your knowledge, but you can still manually upgrade when convenient, and other security upgrades won't be impacted.
For completeness, when Firefox handles its own upgrades:
This does not really apply to this question, since Firefox's built-in update mechanism doesn't suffer from the problem described (where new tabs refuse to work until Firefox is restarted). But for the sake of completeness, on systems where Firefox handles its own updates (e.g. Windows), here's the about:config options to control it:
app.update.enabled - When set to true, Firefox checks for updates automatically. When set to false, it does not. The default is true.
app.update.auto - When set to true, updates are automatically downloaded and installed when available. When set to false, Firefox asks the user what to do when an update is available. The default is true. (This assumes app.update.enabled is true, because otherwise, it won't check if updates are available at all.)
These entries may or may not exist on your system. If not, Firefox will use the default unless you create them.