Interesting discussion. I haven't really used Safari in years so not familiar with how they do it.
I will say when I am looking for something like extensions or other (small s) settings I will look in menus on the far left or the far right. Thats just usually where they are. Having extensions in with other (small s) settings like Preferences makes logical sense to me. I could also logically see them under the Tools menu because in a way extensions are tools.
If I was a developer I could understand why extensions would fit under Library, but as a user Library is for media. It was one of the last places I looked for the extensions.
That said the logical grouping of extensions with History/Bookmarks/Passwords makes a ton of sense. Those things do all sort of go together. However, I do not feel they belong in something called 'Library'. Nor does that belong under the 'View' menu. (speaking of the View menu, why no 'show page source' there?)
So I am sitting here staring at both the Preferences window and the Library window open and the dev in me can sorta see what you are trying to do and it makes sense. The user in me still can't understand why History/Bookmarks/Passwords/Extensions are in something called a Library under the View menu.
I would suggest renaming the Library something else (I don't have a suggestion as to what yet) and then moving that to just under Preferences in the Orion menu. Or at the very least adding a link in the Preferences window to the Library.
Basically my ultimate goal here is to somehow make the Extensions easier to find.