It is great to see this option available after Firefox got rid of their sidebar option when they released a previous update. No idea why they got rid of the old sidebar as it was better than this sideview. With sideview You have to remember to right click the bookmark and then press open in sidebar. With the older Firefox sidebar you just pressed the bookmark and it would open in the sidebar. Its just a small thing really but when you are used to that workflow it takes a while to get into this new way of working.
All sideview needs is a checkbox option to state that when this bookmark is pressed it will always open the bookmark in the sidebar.
It is convenient to be able to open another browser at the same time in one window, but I want you to be able to enlarge the frame size further and return to the previous page. It will be okay if it improves even it. (I'm using a translation machine)