I don't blame you for being confused. There are over 2 million hits for "furnitures". It is difficult to say that all of them are errors. Some of the listings look intentional. That said, I suspect a quirk in Google. It probably has an algorhythm that ignores final s's so one doen't have to search twice for "golf ball" and "golf balls".
From my standpoint, I have never heard or seen "furniture" pluralized. Webster's Third lists an plural "s", but that might be an old use. You certainly will never have trouble if you leave off the s.
I searched the British National Corpus and it only has one example and that's an error: and everything they need to know about the furniture well pieces of furnitures.
They have 3456 examples of 'furniture'. If you look closely at the Google results, you'll see that some are probably mistakes: The best shop for kid furnitures,. Baby furniture, kid games, child furniture and toys store.
Here, the plural is used once and there doesn't seem to be much reason for it.
This one is aimed at catching search engines:
We have been specialized in collecting, restoring and exporting custom Chinese antiques, chinese furniture, chinese furnitures, antique chinese furniture,
They are trying every combination possible as search terms.
Some are not English: Antiques and furnitures. Glacier Familie Walter Perren Matterstrasse 59 3920 Zermatt 4E.
Also, Google shows the singular sometimes when you search for the plural. I have seen the plural used to describe all the furniture in a place, though 'furnishings' would be a better choice. I am not saying never ever, but 99.9% of the time, the plural will be wrong. Google is a useful tool for searching for language, but remember it will show errors and non-native sites ate the same time. I make a lot of typing mistakes- Google will show those, but it doesn't make them correct.
If you want to search for language examples, here's an excellent site: [DAVIES] Variation in English Words and Phrases (VIEW)
It searches the British National Corpus, which is a huge database (we're still waiting for the American corpus, but it is on its way). As well as being able to search for words and phrases, the words have been tagged grammatically, so you could search for a word + gerunds or prepositions, and you can choose the type of text you want to search. I use Google a lot for quick searches, but I also use this site as well, and think it's an excelent tool, and one designed specifically for studying language. ;-)
It is, however, the fastest tool for searching the database. there are some others, but they are slow and not as easy to operate. By the way, if you use Firefox, you can increase the size the size of the font from the View menu, then open Text Size and enlarge it. ;-)
But unfortunately, I don't use firefox, I use Windows and Explore. I can change the text size, but it's still so small to me even I increase the size to the maximum. But anyway, Thank you all the same.