I have a 2002 Ford Focus with about 83,000 miles on it. It is a manual transmission and has been with me since it was 15k miles old. Recently, I took the car to Firestone/Bridgestone to have new brakes put on. They ended up putting the front passenger brake on backwards so that a large paper clip looking thing (I, obviously, am not a car expert) kept poking out and hitting the rims. I took it back and they fixed it but now I notice a clunking noise coming from under the floorboard on the passenger side. When I sit in the passenger seat I can actually feel it. It is most prevelant at lower speeds when I am going straight. If you have any insight I would appreciate it! Also, I am about to drive it 450 miles and am hoping the wheel doesn’t fall off, let me know if that is something to be concerned about 
The most common problem with these suspensions would be the swaybar links. Nobody makes good quality links for OEM, in my opinion… but the replacements are not that expensive - some aftermarket for the Focus are as low as $10, but you can get very nice Moog ones for $25ish. When they go bad, you’ll notice it more at low speeds, particularly in situations like moving straight but hitting uneven bumps - speed bumps are perfect to test these out on.
But there are FAR too many things that could be going bad that are serious safety problems - you really should take it into a professional mechanic (not a firestone place) to have it inspected. It could be cheap and not a big deal… but don’t risk it.
ditto on the “firestone place is NOT a professional mechanic” thought.
in all likely hood it is a secondary issue, which you just started to notice since the brake job, but hey… if firestone place missed the caliper backwards, i would not have much faith in them either to figure this out.