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