If you're using the PortableApps.com Platform, rename your FirefoxPortableESR directory to FirefoxPortable and then check for updates.
If you're not using the platform, rename your folder from FirefoxPortableESR to FirefoxPortable then download the current copy of FirefoxPortable and upgrade in the usual manner (manually install the app over itself).
Sometimes, the impossible can become possible, if you're awesome!
If you're using the PortableApps.com Platform, rename your FirefoxPortableESR directory to FirefoxPortable and then check for updates.
If you're not using the platform, rename your folder from FirefoxPortableESR to FirefoxPortable then download the current copy of FirefoxPortable and upgrade in the usual manner (manually install the app over itself).
Sometimes, the impossible can become possible, if you're awesome!
So easy? Great, many thanks!
Windows 10 Home, 64bit