Is there a keyboard shortcut to enter Reader View in Firefox?

You can use Alt+V, then R to open the Reader View quickly through the menu bar.

These shortcuts may be different in non-English versions of Firefox. Just figure out what the equivalent shortcuts are for (in the main menu) View → Enter Reader View.

---

As of Firefox 50, you can also use Ctrl+Alt+R (or Cmd+Alt+R on a Mac).

Yes there is, as of Firefox 50.0 (November 15, 2016). This shortcut was just added with the release of Firefox 50.

Ctrl+Alt+R (command+alt+r on Mac)

Source: 50.0 Release Notes

Is there a keyboard shortcut to entering Reader View in Firefox?

Short answer:

No.

Long answer:

There is an open (unassigned) bug report requesting that one be added:

Source Bug 1144749 - Create reader mode keyboard shortcut