Original or translated into modern French? The former isn't exactly rare (I think there are 2 current paperback editions with the middle French text), but sadly it's not that common for people to read Montaigne in the text, even though it's worth it for anyone interested in languages (it's much easier than Rabelais).
There are also halfway editions, with only the spelling modernized/standardized but with the middle French syntax/grammar and archaisms left intact.
The modern text is also worth reading, for the content. It's often people's "fall back book", read in small doses between other books or as pillow book.
That being said, I also have an e-book edition of all three volumes that is a copy of the 1580 edition. That one certainly has some interesting orthography.
Je suis méchant.
*NM*