(Second browser-specific - Firefox - bug I file today after 54322).
It was reported by Fram at en.wp, quickest way to duplicate:
<<cursor in front of ref, use the "back arrow" on your keyboard once (so the ref number gets "blue"), and type.>>

https://en.wikipedia.org/w/index.php?title=User%3AElitre_%28WMF%29%2FSandbox&diff=575562298&oldid=575557792 (with a snowman as well).

More ways:
<<Type some text, add a ref, remove the ref using backspace, then change your mind and use the "undo" arrow of VE. No, change your mind again and use the "undo" arrow once more, again removing the ref. Now type. Notice anything strange? Yep, I'm typing backwards. https://en.wikipedia.org/w/index.php?title=User:Fram/sandbox&oldid=575553019
[...]
Other tests produced the snowmen, or the insertion of huge chunks of text by VE. https://en.wikipedia.org/wiki/In_My_Dreams_%28Emmylou_Harris_song%29?veaction=edit
[...]
Easy way to check the "backwards typing": go to https://en.wikipedia.org/wiki/In_My_Dreams_%28Emmylou_Harris_song%29?veaction=edit , put the cursor after ref 1, backspace, undo arrow, type.>>

TeamGale adds:
<<I tried to reproduce it because I was curious and it's a really weird bug! Although when I put my cursor at the end of the "backwards typing" and clicked space, I could type straight again! TeamGale 10:03, 3 October 2013 (UTC)>>

Version: unspecified
Severity: major
See Also:
https://bugzilla.wikimedia.org/show_bug.cgi?id=57600
https://bugzilla.wikimedia.org/show_bug.cgi?id=61538