In a large pot over medium heat, stir together the zucchini, apples, raisins, currants, vinegar, orange juice and zest, brown sugar, white sugar, cinnamon, allspice, cloves, nutmeg and salt. Stir until the sugar has dissolved, and allow the mixture to boil. Reduce heat to low, then simmer until thickened, 2 to 3 hours. For a thicker mincemeat, stir in the bread crumbs; simmer for 5 more minutes to absorb excess liquid.
Sterilise the jars and lids in boiling water for at least 5 minutes. Pack the mincemeat into the hot, sterilised jars, filling the jars to within 0.5cm of the top. Run a knife or a thin spatula around the insides of the jars after they have been filled to remove any air bubbles. Wipe the rims of the jars with a moist paper towel to remove any food residue. Top with lids, and screw on rings.
Place a rack in the bottom of a large stockpot and fill halfway with water. Bring to a boil over high heat, then carefully lower the jars into the pot using a holder. Leave a 5cm space between the jars. Pour in more boiling water if necessary until the water level is at least 2.5cm above the tops of the jars. Bring the water to a full boil, cover the pot and process for 10 minutes.
Remove the jars from the stockpot and place onto a cloth-covered or wood surface, several cm apart, until cool. Once cool, press the top of each lid with a finger, ensuring that the seal is tight (lid does not move up or down at all). Store in a cool, dark place.