can't edit or add bookmark on Safari in Macbook
In the past week since last week, I have tried to add a new site into my Bookmarks or edit the address for one of the sites on my Bookmarks but I can't. It has kept loading slow! I had to click on the Safari icon to hit the "Force Quit" and close it down. Of course, I did report the problem to the Apple company numerous times every time when I hit the "Force Quit" to close it down and re-start it. I had rebooted my Macbook a few times but it did not work either. My Macbook is six years old and appear to be getting slower than I used to have in the past. My Macbook's OS version is provided in the snapshot attached. Thank you for your attention.
MacBook, OS X El Capitan (10.11.2)