DNS requests are normally done to dedicated servers - each time you type an address, a DNS request is made to find the corresponding IP address. The server used for each DNS request can be your ISP's default one, or any one you choose - and it can be changed, per program too. In this occasion, Firefox will by default send DNS request to specified Cloudflare servers - Firefox does not have all the IP's of the numerous internet addresses stored inside the program! Moreover, these IP's are bound to change periodically - what is constant is the domain name after all! That's why, although your PC may store some IP addresses corresponding to domain names for faster resolving, that's only temporary - after a while, you will need to send DNS request in the wild again to get the proper IP's.

In short, the router - and your lower level network system in your PC, too - knows only of IP addresses. The job of DNS requests is to get an IP address to work with, when you want to contact a specific domain.