Firefox Addons fix on Ubuntu 18.04

So after Mozilla fucked up extensions aka add-ons for their Firefox web browser (if you haven't noticed, you may stop reading now), they now released an update to fix that bug. After updating your Firefox browser, your add-ons should be back and operational once again.

To get this bug fix, you want Firefox version 66.0.4.

Unfortunately, running the Ubuntu Software Updater did, in my case, not actually update my Firefox browser.

Running sudo apt update, followed by sudo apt install firefox only informed me that I already had "the most current version".
firefox --version confirmed that my "most current version" was in fact Firefox 66.0.3.

It seems that at the moment (May 7, 2019), the Ubuntu repository does not (yet) have the Firefox release featuring that bug fix.

But there's hope!

Install Firefox 66.0.4 from Mozilla PPA repository:

1) Add Mozilla signing key:
sudo apt-key adv --keyserver keyserver.ubuntu.com --recv-keys A6DCF7707EBC211F

2) Add Mozilla repository:
sudo apt-add-repository "deb http://ppa.launchpad.net/ubuntu-mozilla-security/ppa/ubuntu bionic main"
sudo apt update

3) Install Firefox (this will now obtain the latest Firefox version from the Mozilla repository you've just added):
sudo apt install firefox

4) For confirmation:
firefox --version returns
66.0.4

5) Launch Firefox to see all your beloved add-ons restored to their former glory.

The above instructions, namely details about Mozilla repository, taken from here.