Many jams call for a squeeze of citrus, such as lemon, to balance the sweetness of the fruit, but vinegar can bring the same acidity with a more neutral flavor. In this jam, a small bit of red wine vinegar balances the sweetness but lets the strawberries be the star.
Combine the strawberries, sugar, vinegar and salt in a medium saucepan or Dutch oven. Toss well and let stand until the strawberries are juicy and the sugar is moistened, about 20 minutes.
Add the elderflower liqueur to the pan and bring to a boil over medium-high heat, stirring. Reduce the heat to maintain a simmer; cook, stirring and skimming off any foam, until the juices become glossy, thick and syrupy and the fruit is very soft, 25 to 30 minutes. If the berries are still in large pieces after about 20 minutes, gently mash with a rubber spatula or potato masher. Remove from the heat and let the jam cool to room temperature, stirring occasionally, about 1 hour.
Transfer the jam to a jar or other airtight container. Refrigerate until cold and fully jelled, at least 2 hours. Store in the refrigerator for up to 2 weeks.