i would recommend selling it, cb is pretty useless now a days...
if you insist on keeping it though, 4 foot firestick is the way to go, just make sure to get a spring mount and maybe a quick disconnect
Useless? No.
I had one in my Ranger and used it on a regular basis. I'd get out on the highway (mostly traveling to and from college) and just tune that thing to channel 19. There was a period of time where I just left it off, but after getting stuck in major traffic because of a wreck up ahead or the like, I started having it on every time I hit the highway. Truckers tend to pass the word when things like that are up ahead and if you know how to ask, they'll tell you if there is anything like that up ahead as well as how to best get around it.
I put one in my choptop as well. It hasn't really seen highway use but most of the trail rides I've been on used a CB for communication between trucks on the trail.
Granted a CB is not as extensively used as it once was, but it is NOT totally useless.
On my Ranger I ran a fiberglass whip (similar to a Firestick). While I had my cap on, it was attached to the ladder rack centered side-to-side and right behind the cab. Then I tuned it in the best I could. When the cap was off, I either used a magnetic mount on the roof or rigged up a fiberglass to the bed.
Just randomly mounting an antenna can produce less than steller results. The antenna needs a good ground at it's mounting location because it gets it's effective range by bouncing signal off of the metal body of your vehicle (also known as ground plane). The more of your antenna "blocked" by the body, the shorter the range (such as mounting it to the top of the bed right behind the cab, you can essentially turn a 4' antenna into a 2' antenna as far as forward signal). Ideally, you want the antenna placed in the middle of the largest "flat" panel you can if you want max signal in all directions. Place it towards a corner of that for max reach in the longest direction. Longer antennas also help with longer signal distances.
My choptop sports a 102" stainless whip. I did that partly because I don't trust fiberglass to hold up to off-roading and partly because a 102" whip will give me the most reach since it isn't really practical to mount the antenna to the center of the hood (the largest flat piece of steel) since it's a choptop. I mounted it to the spare tire bracket so it's nice and high. I had a 102" steel whip attached to the same spot when I had my 89 Eddie Bauer Bronco II and used it on the highway several times. It worked out quite well.
I actually miss having use of a CB in my F-150. I have one I can put in there, I just haven't got around to installing it and rigging up an antenna yet.