On tiling window manager, "site info" uses new window instead
I use i3 too and sometimes also Firefox opens new windows instead of popups.
I think this is more related to i3 rather than software which runs on your machine
Anyway welcome to the forum!
Yes, it is normal i3 behavior, which use stacked instead of floating window. But, I think this problem is on Vivaldi. Clicked on the same button in Firefox and in another chrome based browsers are popup instead of new window like Vivaldi did.
Did you see my link?