When Eva was first diagnosed with diabetes she was twenty.
When Eva was firstly diagnosed with diabetes she was twenty.
Which one is grammatically correct to say and as I know both first and firstly we can use as an adverb.
When Eva was first diagnosed with diabetes she was twenty.
When Eva was firstly diagnosed with diabetes she was twenty.
Which one is grammatically correct to say and as I know both first and firstly we can use as an adverb.
Both appear to be correct as adverb form of firstly appears to be a synonym of adverb form of first. https://en.m.wiktionary.org/wiki/firstly
However, I saw only first being used the way you used it. The second form using firstly looks clumsy to me but it may just be that I am not used to it.
I only saw firstly used in the sense of "in the first place".
If you have no specific reason to prefer firstly, perhaps you should prefer using first to avoid any confusion.