You can, but we can't be sure if they will play nice. Think of it like this, Built-in VPN will mask your browsers traffic, while your stand-alone VPN will protect your devices traffic. Hope this helps!

Depening on where you are located it may not be available for you to use yet.

https://support.mozilla.org/en-US/kb/built-in-vpn https://blog.mozilla.org/en/firefox/built-in-vpn/ https://blog.mozilla.org/en/firefox/firefox-148-149-new-features/

This "VPN in Firefox" feature is experimental and is being introduced to the Firefox user base through a progressive rollout. It may not yet be available to all users.

Users will have 50 gigabytes of data monthly in the U.S., France, Germany and U.K. to start. Available in Firefox 149 starting March 24.

So mozilla give some users this new feature that those who provide support do not have?!?!?! Is it just on windows or what?