I do not think fire sticks are cut for a spring. It will make the antenna electrically long. This means your best swr will be on channel 40, and your worst on Channel 1. Try it and see what you can get, but it should theoretically be better without the spring.

You can buy a 3’ jumper cable and barrel connector to get the 12’ up to 15’ for the easiest plug and play. Added bonus: you’ll be all set to add a linear or external swr watt meter with this config.

Pro Trucker Female to Female SO-239 Coax Connector Barrel Coax Adapter https://www.amazon.com/dp/B01G2R5JY6/ref=cm_sw_r_cp_api_wNelBb6H8F2TE

RG58A/U COAX CABLE 3 foot Jumper for CB / Ham Radio - Workman CX-3-PL-PL https://www.amazon.com/dp/B005IDVKR6/ref=cm_sw_r_cp_api_iOelBb2F2FADS

You’ll have to send pics. Everything should just screw together. The fire stick should have a stud that screws into the spring, and the spring should have a stud that screws into the base mount. If the spring does not have a stud, then they likely want you to buy their proprietary mount to match their spring. You can get around that by cutting a short piece of 3/8-24 threaded rod and make your own stud.

Edit: The spring you linked to earlier on Amazon shows that it has the male stud that screws into the base mount, and has the female end on top where the fire stick male stud will screw in. It should all be plug n play.