Sorry, but a "very old user of Firefox" is someone who has used it since before Firefox 1.0 which was released in Nov 2004. There are a number of us around there who have been using it since 2002 - 15 years now. Almost 2 years before it was even called Firefox.

No Tablet or "Mobile" versions of Firefox for Windows devices. Mozilla had invested a lot of time making Firefox for tablets and phones only to have Microsoft "change the rules" at a very late date would have required massive changes what had already been done to make Firefox work on that platform. And as it turned out that version of Windows wasn't very popular and "never got much traction" in the market place. So Mozilla was wise to "take a pass" on it.

As far as Firefox being available thru the Windows Store - ain't gonna happen. I don't recall the details, but Microsoft's demands were onerous for open source software like Firefox. And that was about the same time as the new "Mobile" versions of Windows were nearing release.

The regular version of Firefox for Windows will run on tablets that use Windows 10 Pro, but not on Windows 10 S or RT (which, as you point out, are limited to store apps).

Mozilla has its hands full with desktop versions for Firefox for Windows, Mac, and Linux, and an Android version, and its other products, Focus for Android, Focus for iOS, and a Firefox-like shell for Safari on iOS. It's hard enough to keep up as it is!

Firefox's code is all open source, so if someone is motivated to create a Windows tablet version, they certainly could. They just can't call it Firefox.