I’d absolutely recommend using your own modem and router. I do it and the additional control over your network is more than enough reason to make the switch. Plus, using your own modem, and router usually pays for itself in two years, so there are financial benefits as well.

You will need to make sure you get the right modem for your ISP. You can usually verify which one you need on their website.

I found the binary for the sid repo. That worked. Here is the link in case someone does a search: http://ftp.de.debian.org/debian/pool/main/f/firefox/