Two instances of Firefox in the same installation

I have found a strange problem that I don't know how to solve.
Here it is:
I had an instance of Firefox portable in my computer. I deleted it. I deleted the folder in "AppData/Local" also. I deleted all the related files I could found. There was no any installation whatsoever of Firefox in Program Files. I downloaded Firefox Portable 25.0 and installed it. Strangely enough, when I initiated Firefox portable from the .exe, the new Firefox appeared, but when I tried to open an html page, another Firefox (the ancient Firefox) appeared, with all the old tabs open. And that was really driven me crazy. So I opened Task manager and found that this "second" Firefox was inside the new one, exactly in the "App" folder. I found in this "App" folder the following folders:
AppInfo
Bin
DefaultData
Firefox

And inside this last "Firefox" folder was all the machinery of the old Firefox. And when I erased this folder, an error message popped up, saying that Firefox could not function. So I delete again all the files and folders related to Firefox. I put the Firefox installer in an external hard disk and installed it from there into the desktop. And the same thing happened again! Two instances of Firefox in the same installation, the way I have described up here.

Questions are:
1. Have anyone seen something like this before?
2. How to solve it?

I would be very grateful for any help.