Did you try "searching" for remaining files?
If you previously tried to uninstall it, I'm wondering if you may only have "bits and pieces" left. Without knowing what steps you took to remove it, or (specific) problems you're experiencing now, I'm at somewhat of a loss.
Ordinarily I might suggest installing a new version "over the top", with the intention of removing it, but I don't know what state FF is presently in. It would be important to know, if the firefox.exe is still running in the Task Manager, under the Processes tab.
After "unhiding files and folders", did you "search" for all Mozilla and Firefox related files? Does the C:\Program Files\Mozilla Firefox (or C:\Program Files (x86)\Mozilla Firefox for 64-bit) folder still exist? What about in C:\Users\<your user name>\AppData? Does your Profile still exist? This shows you "how and where" in Windows 7.
I really don't know what else to tell you. I've used the trial version of the Revo Uninstaller and never had a problem. You're not going to be forced to pay anything. And it's not going to remove any files, other then those related to Firefox. For all I know, Revo Uninstaller may not even list FF, if most of the files have been removed.
I have a couple of other ideas, but first try removing any remaining files.