First, you don't need exactly 18' of coax cable, buy the length needed with a little extra for good measure & you'll be fine. The 18' length thing is an old rumor that is nothing more than an old wive's tale.
A ground strap is only needed when the mount can't be grounded adequately. If you grind the paint off the tub & the powder coating off the Teraflex mount where they bolt together, you'll have an adequate ground & no ground strap will be needed.
You'll need a mount for the antenna so it can be mounted to the Teraflex body mount. A 3' antenna mounted at tail light height won't always clear garage doors so if that is an issue, you may be better off with a 2' antenna. I converted my Firestick FS-3 to an FS-2 so it would clear my garage door and it works fine & gets out well on the trail.
Oh, make sure the Firestick antenna you buy is one of the FS series which means it is easily tunable via a vinyl-covered brass screw on its tip.
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Yes, they mean a firestik K4 or K4A. One accepts a lug style ring connector and the other accepts a PL-259 style threaded collar. Both have the same antenna atachment method, it just differs in the type of coax you run to it. Make sure they are compatable.
FYI, an SO-239 is the plug that a PL-259 screws onto. Your radio, for instance, has an SO-239 plug, and your coax will have at least one PL-259 to screw onto it.
Let me second what Jerry said about install - make certain to insulate the upper shaft by properly using the nylon washer. If you don't it will not work right, if at all. If you use a lug style mount, both sides get insulated.
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