https://support.mozilla.org/en-US/kb/custom-installation-firefox-on-windows

You will want the full offline setup and not the small online stub installer. https://www.mozilla.org/firefox/all/#product-desktop-release

James, much thanks. That is very helpful.

Do you know if Automatic Updates of the custom installation will end up where I want them, or will I have to manually repeat the custom installation process as updates become available?

Note -- from https://support.mozilla.org/en-US/kb/custom-installation-firefox-on-windows we read: "In the next screen, you can choose whether to install the optional Mozilla Maintenance Service which allows application updates to occur in the background, without requiring you to click [Yes] in the Windows User Account Control (UAC) dialog."

I can clearly disable this. But, if enabled, where will the update go?

Normally, Firefox establishes two .ini files on your user profile folder. These .ini files can point to profiles located on other drives, but I am not aware of a way to move the .ini files themselves to other drives.

There is a third party rebuild of Firefox intended for removable media that is more self-contained. You could take a look:

https://portableapps.com/apps/internet/firefox_portable

Well, I tried the MSI and did not see a way to get it to install on E:. I could download to E:, but not install.

As installed on C: (without porting my profiles) it loads C: with about 400 MB.

So, I'm wondering if I need to bite the bullet on portableapps.com. That would help me with: Firefox, T-bird, GIMP, and maybe a couple of small footprint apps (e.g. VLC; WizTree already has a portable version). Maybe I can pick up some new things to replace Adobe, which is a serious data space hog.

Thanks all'ya'all.

Well, I tried the MSI and did not see a way to get it to install on E:. I could download to E:, but not install.

Select the Windows 64-bit in drop menu list and not the MSI to get the Firefox Setup 127.0.exe to do custom location install.

Portable apps version of Firefox is basically the official build but with a custom launcher to keep the Profile self contained with the Firefox folder so you can say even run it completely off a usb flash drive for example. They have been doing this for a couple decades now with permission from Mozilla.