You should see both entries in the Add/Remove programs section of Windows. If you prefer to keep using the 64-bit version that you probably installed at some later point, you can safely remove the 32-bit version that shows (x86) in that dialog.

In any case, do not "uninstall" just by removing the content of the folders you mentioned. Best choice is always to use the dialog mentioned.

If you only see only one entry listed in Add/Remove programs, that’s the one that should be "known" by your OS, probably the 64-bit version you seem to use now. In that case, you could choose to remove the one from the Program Files (x86) folder instead. To be safe, rename the folder first and see if your current Firefox instance doesn’t break, then remove it.

Problem solved, only one version now installed

Thank you, Mozilla! The above info fixed! I have Firefox 57.0.2 only. You're the tops!

I wanted to add my thanks to this thread.

The same problem popped up for me, causing FF 59 to keep installing even though I had configured FF 58 NOT to automatically update.

In my case I was able to find two FFs in Control Panel: one for FF 58 & one for FF 59. I uninstalled FF 59 through the control panel, but was left with FF file folders in both the Program Files and Program Files (x86).

I checked to make sure that the Program Files (x86) file folders didn't contain anything relevant to FF 58 by renaming them. The result was that FF 58 functioned just fine. So I then went in and manually removed the file folders in Program Files (x86). 

I hope this ends my issues with FF 59 automatically updating & being incompatible with some of the websites that require me to dl information.