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You will not be able to delete inserted items in Firefox inside a contenteditable element using backspace.
contenteditable
Almost all browsers (including IE > 7) can delete the inserted items using the backspace key, but not Firefox.
This is an open bug on Mozilla's bug tracker:
Bug 685445 - Contenteditable=false elements in contenteditable=true elements cannot be deleted by using Backspace
The user has to select the whole item to delete it.