Hi kstrygg,
I don’t recommend doing business with Foam By Mail (or any of their other websites) unless you are OK with lower quality materials or where its not critical that what you receive is what you ordered. They are not a reliable supplier IMO.
You can see more of my thoughts about them in post #2 here as well as post #2 here (and many other posts around the forum that say basically the same thing).
As a general rule … 50 ILD is very firm (firmer than you will usually see as a base foam) but I don’t particularly trust their specs anyway.
Phoenix