Can't confirm the bug.

I've a different behavior with Firefox 15.0.1: All the content is deleted except a sentence (currently "Also, there strange chess symbols appear....").

@Dereckson:
I just re-tried and you are right.
I changed the bug's title but can't change the description unfortunately.
Backspace has the exact same problem, contrarily to what my description was saying.

Okay, thank you to have rechecked.

I guess we should settle on "Unconsistent behavior selecting all then deleting content".

[ Confirming bug. ]

Fixed here: https://gerrit.wikimedia.org/r/#/c/28790/