How to move focus back to the webpage in Firefox?

The answer depends on whether you typically have a sidebar open. tl;dr:

• Tab (if no sidebar is open, or if sidebar is on the right)
• F6F6 or ShiftF6 (if sidebar is on the left)
• CtrlF and then Escape (works regardless of sidebar position, avoids problem with page scroll position jumping around)
• The javascript: shortcut hack from this answer also works in Firefox: https://superuser.com/a/324267/143467

In Firefox 57 the search bar is gone by default. If you do not have a sidebar open then pressing Tab once returns focus from the address bar to page content (as @DavidPostell and @xuhdev suggested in the other answer). But using Tab often has a side-effect of causing the page scroll position to jump around.

F6 (Move to Next Frame) also works. I had hoped that F6 would avoid making the page jump around, but it turns out that is not the case.

If you use a sidebar then things are more complicated. I usually have the Tree Style Tabs sidebar open, and I have to press Tab four times to get focus back to the page content. I can press ShiftTab twice, but that has a side-effect of scrolling the page to the bottom. On the other hand, pressing F6 twice reliably cycles to the sidebar and then to the page.

You can move the sidebar to the right side of the browser to change the order of elements/frames that F6 and Tab cycle through. If the sidebar is on the right, then pressing F6 or Tab once moves focus from the location bar to the page, just as if there were no sidebar open.

My ideal would be to customize Firefox so that pressing Escape moves focus to the page. But I have not found a way to do that.

What keyboard shortcut can I use to move the focus back to the webpage?

Alt+D , then press Tab twice (or three times):

Get "focus" on webpage for in-page keyboard navigation. Note the dotted frame around the page.

Source 82 Shortcuts for Firefox (Windows)