That sounds like Content Blocking kicking in - that's only active in Private Browsing by default: https://support.mozilla.org/en-US/kb/content-blocking

I note that page says "By default, content blocking uses the Disconnect.me basic protection list" - and reddit.com is on that list: https://github.com/disconnectme/disconnect-tracking-protecti...

(I'm guessing reddit's "social button" is considered a tracker.)

[edit] confirmed, it's definitely Content Blocking: I just loaded that jsbin in an FF private window, and there's a message in the console to that effect.