I'm not entirely sure what you mean with that, so I can't test it, but have you tried about:blank? So, just give it "about:blank" as URL in New Tab Override.

That's a blank page, as you might have guessed, but it's built into Firefox, so should load a lot faster. Really don't know, though, if Firefox actually treats it like a webpage or not.

Extension defined pages and all special "about:" pages are apparently off limits. You also can't make New Tab Override point to a local file (it can load a local file and present it as a webpage, but that defeats the point as it just becomes an extension defined webpage again).