That's because Google sniffs Firefox's user agent and serves it an older, less featureful version of Google Search. One that doesn't support AMP. Spoofing Firefox's ua-string to appear as Chrome will get you the full Search experience, including AMP.
But according to some on here a no AMP response with FF is better as no AMP. Which is it?
Personally prefer the faster speed with AMP.
I do use Google, although I don't search for news much. I usually find NYT, Guardian etc articles from their homepages.