how do I turn on VPN

If you have the built-in VPN option, there should be a VPN icon in the top-right of the Firefox toolbar. Click it, choose Get started, sign in to your Mozilla account, and then hit Turn on VPN.

If you don’t see the VPN icon at all, it may simply not be available on your Firefox yet since Mozilla is still rolling the feature out.

Bonsoir

Use built-in VPN in Firefox https://support.mozilla.org/en-US/kb/built-in-vpn

Unable to find the built-in VPN in Firefox https://support.mozilla.org/en-US/kb/unable-find-built-vpn-firefox

Otherwise, please, check, the preference, browser.ipProtection.enabled https://support.mozilla.org/en-US/questions/1578173

To complement, see also this discussion thread https://support.mozilla.org/en-US/questions/1605682