How can I downgrade Firefox 41 to Firefox 40?

In the latest post about the Incompatibility between the Norton Toolbar and Firefox 41, Athena suggests:

"If you have already updated to Firefox 41, consider downgrading to Firefox 40."

Now that we've learned that it will be at least a month until Norton Toolbar is re-written for Firefox 41, I would gladly go back to Firefox 40.

Does anyone have a suggested method for doing this?  I already tried running the installer for Firefox 40.0.3 for Windows - that doesn't work. The Firefox installer only upgrades, it does not downgrade to a prior version.

And I certainly don't want to remove the Firefox application altogether with all my bookmarks, add-ons, etc. - in order to do a clean re-install of FF 40.

So HOW EXACTLY does one downgrade Firefox 41 back to 40?