Since 2019, Firefox have been rolling out
DNS over HTTPS (DoH) by default in several countries, including the USA, Canada, Russia and Ukraine.
What does this mean?
When DoH is enabled, it bypasses your DNS server and instead, domains you enter into your browser are sent via a DoH-compatible DNS server using an encrypted HTTPS connection.
This is intended as a security measure to prevent others (e.g. your ISP) from seeing the websites you are trying to access. However, if you're using a DNS server provided by your VPN gateway, it allows DNS queries to run outside the VPN tunnel. Moreover, if the VPN specifies a DNS server that resolves internal host names, these are either not resolved at all or resolved incorrectly when DoH is enabled.
How to disable DNS over HTTPS in Firefox
To ensure all your DNS queries run via your VPN's DNS, you will need to
disable DoH in Firefox. To do so, open your Firefox browser, go to Firefox > Preferences > Network Settings and deselect the checkbox by "Enable DNS over HTTPS":

Click OK to save your changes.