Combine strawberries, rhubarb, sugar and salt in a heavy 6-quart pot; bring to a simmer over medium-high heat, stirring often. Reduce heat to medium; simmer, stirring often, until the mixture registers 220 degrees F on a candy thermometer and becomes thick and slightly darker, about 1 1/2 hours. (Alternatively, to test for proper thickness, place a small amount of jam on a chilled plate; place in freezer for 2 minutes then run a finger over the jam. If it wrinkles and feels gel-like, the jam is ready. If the jam is not yet ready, continue cooking for 5 minutes, then test again.) Remove from heat; stir in lemon juice.