What's the best way to configure Firefox for use with a slow internet connection?

How do you mean slow? The browser works at the ISP bandwidth provided you can't exceed that. To keep the Browser from being bogged down you shouldn't load toolbars, Addons, Extensions or make a Homepage that loads excessive content. This will keep the Browser from being bogged down when running. Otherwise a clean Browser with nothing added is the DIY to keep the Browser from being slowed down. Last resort is to get a higher ISP speeds to be able to view more contents. And last I make Google.com as the Homepage as that is a clean display nothing added unlike a Yahoo.com homepage that has to much loaded on it from the get go.

My internet connection is slow. It's through a phone line and the speed is like 670. I remember a long time ago the settings in a browser could be changed to stop the download of big graphics and pictures. Can Firefox do this for me?

Yes. Preferences > Content and uncheck the "Load Images Automatically" box

and set your browser to mobile device using this extension

https://addons.mozilla.org/en-US/firefox/addon/user-agent-switcher/

Both of the above tips from https://lifehacker.com/5791586/how-and-why-to-set-up-a-secondary-browser-optimized-for-slow-internet-connections

However if your Internet connection is that slow, then a text only browser may be the way to go. E.g. Lynx http://lynx.invisible-island.net/ list here https://merabheja.com/12-text-only-browsers-for-browsing-in-slow-internet-connections/

There are extensions that can block content.

• https://addons.mozilla.org/firefox/addon/policy-control/
• https://addons.mozilla.org/firefox/addon/ublock-origin/