Several versions ago, Mozilla introduced a long overdue new tab page which displays thumbnails of often visited sites, speed dial style, similar to Chrome's new tab page. It's a huge step forward from the blank tab that used to present itself when opening a new tab, and it can even be customized with an extension or some about:config tweaks.
For some, though (including yours truly), it wasn't enough. I still prefer to have a custom new tab page load. Besides those which overhaul the new tab completely, there are several addons available which allow you to load a url of your choosing in a new tab, but I'm always on the lookout for ways to eliminate addons and keep my Firefox installation as light as possible.
So I was thrilled to find that you can change Firefox's new tab page without having to install an addon. Here's how to do it.
Just punch in "about:config" in the browser's address bar and click "I'll be careful." In the search bar on the next page, type in "newtab" (with no spaces). The first option will be "browser.newtab.url." That's the one we want, so double-click on it. Now, simply replace what's there with any url or local file address you want.
While you're digging around these preferences, you may want to take note of the other preferences in that window, which allow you to customize the default new tab I mentioned earlier, without installing addons. If you want to add a background or otherwise style the page, you'll need the extension mentioned above, but these settings at least allow you to set the number of thumbnails that the new tab page displays.
And that's it. Now, new tabs will automatically load your chosen page, and your system's just as light as it was when you began.