Unfortunately, some users may encounter the error "failed to delete note" when attempting to delete especially old or imported content via a web browser such as Firefox, Internet Explorer, or Chrome. Worry not! I have a solution for some of you, and it worked more recently than two years ago.
Here's what you need: A smartphone and the Facebook App.
Got it? Good. Let's get started
First, you want to open the Facebook App and travel to your profile's home screen. It's the one with your cover photo, wall and etcetera. Once there, you need to scroll to the right a bit in search of an icon which appears similar to this:
Find it? Good - select that sucker!
We should now be looking at a page containing the note or notes you wish to delete. In my case, it's just this one note I wish to delete, but you may have more. Don't Panic because the images are not 100% similar. Your screen will appear more-or-less like this:
Choose the note you wish to delete by selecting it's title (boxed in red). This will take you to the content of the note itself. Once there, scroll to the bottom of the post and locate the "Delete" button:
Once you select "Delete" you'll be taken to a final "Are You Sure" page:
At any rate, this is all it took for me to avoid the "failed to delete note" error occurring on my browser (Firefox) I hope it works for you as well!