What is "zio->i"?

FreeBSD 13.1-RELEASE-p5, using SCHED_4BSD.

What is "top" telling me when a process is in "zio->i" state?  Every
so often, either firefox or thunderbird seemingly sits there
contemplating its own navel in that state for up to a minute or two.
It recovers of its own accord, of course, but I wish I knew what it
meant.  Just a few months ago, I moved my /usr partition from UFS to
ZFS, saving myself some fsck aggravation (rarely necessary, but very
slow when needed).  "top" also reports some pretty big ARC numbers
-- specifically, bigger than free memory, but not big enough to
consume any swap space:

Mem: 1232M Active, 1577M Inact, 27M Laundry, 2675M Wired, 175M Buf, 317M 
Free
ARC: 1823M Total, 283M MFU, 1515M MRU, 16K Anon, 5117K Header, 13M Other
      1712M Compressed, 2135M Uncompressed, 1.25:1 Ratio
Swap: 8192M Total, 8192M Free

-- George