That's disconcerting. Did other Sync data come through, like bookmarks?

You can check what data is in the Sync cloud for your Mozilla Account using the About Sync add-on:

https://addons.mozilla.org/firefox/addon/about-sync/

After you install the add-on, you can type or paste about:sync in the address bar and press Enter to load it. The page shows each category of data currently found in the Sync cloud.

Anything useful under "crypto"?

If not, is it too late to get your data from the old computer (either by running Firefox and exporting, or from the Recycle Bin)?

Yes, my bookmarks did transfer ok.

I dont now if its important, but at first in my new fresh Firefox install passwords were not selected to be sync by default, I had to select it afterward, when my old firefox logins were closed.

In "about sync" I find just 1 record in the "crypto" category, the ID says "keys" (I am not sure how much should I share about all I can see there, I hope thats enough and not too much)

Under crypto, the Records mini-tabs should show whether there are any saved logins in the cloud.

As I said, there is only 1 thing there, called "keys". I'll add some images in case of any help.

Ow, and I forgot to answer before, no, there is no way to boot "the old windows" to try sync there, as it is my current PC, I just did a fresh install as my last one was not fresh and mixing drives did pay a toll in my previous windows.