How to get rid of rogue security?

It's happened to me before. I downloaded Malwarebytes and it just got rid of it. Now, I did a scan on Malwarebytes and it says it got rid of the infections, but the rogue security program is still there. I used rkill to stop all the popups from the program, but it won't let me use Firefox. Whenever I click it, a message pops up reading: "Firefox is either not designed to run on Windows or contains an error. Try installing the program again using the original installation media or contact your system administrator or the software vendor for support.". Help please.