This does not work anymore. In a fresh Ubuntu Mate 22.04 using Firefox 100, the extension says 'App not found', but I have installed the Linux app, I tried to uninstall FF and install it from apt, I have uninstalled and installed a few times, still nothing. Shame, given that you need to pay $100 for a subscription!

I've found ExpressVPN to be a really good service. It's expensive as VPN's go--but for me it's worth the price. The support is superb. Maintaining privacy on the internet is a huge issue and a huge problem--this gizmo helps. I'd recommend it.

Best thing i've ever paid for (ExpressVPN) and what a handy add on, click off a button on or off.

For all that extension ask for an expressvpn app install in the browser extension, this solution i have really works, also in Opera browser:

If you installed your web browser from the Ubuntu Software Center, you may find that the ExpressVPN browser extension is unable to communicate with the ExpressVPN Linux app. To resolve this issue:

Back up your browser settings and bookmarks.
Go to the Ubuntu Software Center and uninstall your browser.

Open the Terminal.

Enter:
$ sudo apt update

Depending on your browser, enter:

$ sudo apt install chromium-browser

or:
$ sudo apt install firefox

Open your browser by entering:

$ chromium-browser

or:
$ firefox

Get the ExpressVPN browser extension by entering:

expressvpn install-firefox-extension

or:
expressvpn install-chrome-extension

ENJOY