As a late birthday present, Mimmi gives Salander "a beautiful cigarette case with a lid of blue and black enamel and some tiny Chinese characters as decoration" (9.43). We wish we could tell you what the characters translate to, but we don't know. When Salander asks Mimmi, she says, "How on earth would I know that? I don't speak Chinese. If found it at a flea market" (9.46).
If we had to guess, we'd say the characters say something about friendship, but even if they don't, that's what the cigarette case symbolizes. When Salander is buried alive by Niedermann and Zala, what does she use to dig her way out of the hole? Why the cigarette case, of course!
This must feel particularly ironic for Salander, who is nursing heavy doses of guilt over the fact that Mimmi was kidnapped and beaten by Niedermann. Now she's using Mimmi's gift to save her own life. This simple, inexpensive gift becomes, at the end of the novel, Salander's most prized possession. It not only reminds her that someone out there really does care about her, but also literally saves her life. It's another example of how Salander isn't alone, despite her Salander-against-the-world mentality.