Selecting "No" to UAC Prompt when updating Firefox does a bunch of bullshit without telling you anything.
If you select No to the UAC prompt when updating Firefox via Installer, it will install Firefox to "AppData\Local\Mozilla Firefox" without telling you, instead of overwri… (daɗa karatu)
If you select No to the UAC prompt when updating Firefox via Installer, it will install Firefox to "AppData\Local\Mozilla Firefox" without telling you, instead of overwriting the copy in Program Files or erroring out. Additionally it will create a new Profile and set this new Profile as the Default Profile without telling you. Lastly, it'll remove an existing Pinned shortcut on the Taskbar.
So you've now got 2 copies of Firefox installed, one of which you're not told anything about and an additional Firefox Profile with no indication that this happened nor any instructions on how to undo this nor how to change the default Profile.