From MozillaZine Knowledge Base
When editing text that includes both left-to-right and right-to-left languages, the delete and backspace keys may have unwanted or unexpected behavior. At intersections of LTR and RTL text, the caret may be on the “wrong” side of a character that would be removed by pressing delete or backspace. This preference controls what happens when delete or backspace is pressed.
Possible values and their effects
Immediately delete the character, even if the caret is not visually adjacent to the character.
Instead of immediately deleting the character, move the caret to the “correct” visual position and allow the character to be removed with a second keypress. (Default)
First checked in
Has an effect in