You can set browser.newtabpage.activity-stream.topSitesRows directly on the about:config page.

You can open the about:config page via the location/address bar. On the warning page, you can click "Accept the Risk and Continue" to open about:config.

• https://support.mozilla.org/en-US/kb/about-config-editor-firefox

That worked great. Thanks. Now. Is there a similar way to add a column and to reduce the spacing? I scrolled through the about:config commands, but didn't see anything obvious to me.

There are not any preferences for the number or spacing of columns. Currently, the layout is designed to adapt fluidly to the zoom level and window width.

If you have a tinkering mindset, it is possible to inject your own style rules into the page using an unofficial, community-support option: a userContent.css file. That's the lesser known cousin of the userChrome.css file some people create to tweak the user interface (toolbars/menus). Since Firefox changes frequently, it can be a challenge to find working rules, but the wizards over on https://www.reddit.com/r/FirefoxCSS/ usually can provide a quick solution.

Thanks, but it is not important enough to get that involved. Thanks for the help.