As adjectives the difference between crumby and crumbly
is that crumby is an alternative spelling of lang=en while crumbly is easy to break into small fragments; brittle or friable.
crumby
English
Adjective
(er)
Crumbly; inclined to break into crumbs.
crumbly
English
Adjective
(er)
easy to break into small fragments; brittle or friable