Is disabling updates really the only way to stop Firefox's "Restart required" on opening a new tab?

Firefox's (seemingly new in 63.0 but maybe I've been lucky until now) background/running updates are causing me problems as I suddenly get "Restart Required" when I open a new tab. FF claims it will reload tabs, but (1) only when I click on them, so I don't notice that I'm no longer getting notifications from ones I haven't clicked on, and (2) Office365 really doesn't play nice when you restart, and that's work, by definition.

So what I want to do is disable background updating. But which of the 30-40 barely-documented options in about:config does that? Does any of them?

This is so problematic that I'll probably just disable all FF updates if I have to, to avoid losing work, by blocking the update server if it comes to it. After all, I've never lost anything due to running an old version (and I have been stuck on old versions for a long time in the past) but I have lost work due to this option.

But surely there's an actual solution. Surely?

There is a solution - or at least a partial solution.

You can change the way Firefox is installed. It sounds like you have it installed through a package manager. If that is the case, then the package manager will also handle the updates for Firefox, which (generally) means it will update even when Firefox is running.

If you install Firefox without using a package manager then it should use its own update feature instead. Once you've installed this way, you may still want to fiddle with Firefox's about:config to get (as close as possible) to the exact update behavior you want.
Note that if you don't change the update method before changing about:config, you may have unexpected results (but I can't speak to the details on that).

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.