Bryan, have you tried redownloading the file?

Yes, several times.

I suspect something else is broken in the system (I've checked the disk, run memory tests, etc.) - but the messages flash by so fast it's impossible to sort out what it might be.

The full setup is at https://www.mozilla.org/firefox/all/#product-desktop-release in case you are downloading from some random download site.

Some antivirus and other security software have caused issues sometimes with the Firefox setup on Windows.

The full setup is basically a self-extracting 7-zip so you can extract it with 7-zip and have Firefox much like the .zip version used to be available for Firefox Releases (for Windows) up till the Firefox 1.0.1 release.

OK, things I've seen/learned: 1. At one point, an attempt to install 127 wiped out the 125 installation but did created an EMPTY 125 install.... This was utterly bizarre. 2. Study of the problem (affecting several installers including firefox, scansnap, and dymo) revealed that "InstallShield" seems to be a common issue (at least for scansnap and dymo) installshield is an installer package, it's not actually part of windows. 3. So I groveled through the system and deleted registry keys, directories, random files, and other misc junk associated with "installshield" AND ScanSnap and Dymo now work.

So i then tried firefox 127, and the installer ran just fine, proper upgrade keeping settings, etc.. (We'll see if it fixes my session crash issues.)

Sadly this isn't very specific advice, and you kind of have to be a windows nerd/expert to apply this. But if people are having installer issues, especially if multiple apps are involved, running down all of the InstallShield droppings in the system seems to help (even with code that maybe isn't using InstallShield....)

See also:

• https://www.google.com/search?sa=X&num=100&q=InstallShield