By "internet browser" I suppose you mean Microsoft Internet Explorer 11, or Microsoft Edge if you're using Windows 10.
A blank page does indeed appear when you "save spacecraft to file" with IE 11. The fact that "Save as" doesn't appear when you try and save as .sss could be related to some browser incompatibility.
While I never use Edge, the ship editor does seem to work correctly with it.
I suggest using a different browser, both latest Google Chrome and Mozilla Firefox work with the online ship editor in Windows 10 and should work in Windows 7 too.
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