Session Boss is on my top 10 list of favorite Add-On (Extensions). It's like a very well organize OneTab, that automatically saves the tabs without closing them.

1) Session Boss has a superb UI!
1a) It has an option to switch from a pop down menu to a full page interface. I've never seen this option in any other Add-On. Great idea.
1b) IMHO, Newbies should use Session Boss with the full page interface at first. It's easier for users unfamiliar with the Add-On to quickly understand and appreciate all its option on a full page UI.
1c) IMHO, The pop down UI is good for quick access, and for those users knowledgeable with the Add-On.

2) I like the scheduled session backup increments, where it saves the last 4 15-min interval backups, the last 4 hourly backups, last 4 daily, last 4 weekly, & last 4 monthly.

3) It has an On-Change backup, which is separate from the interval backups.

##############################################
My Recommended Improvements:
1. Remove hyperlink to "Add-ons Manager", and just display it in normal text. Most users know how to access Add-ons Manager, and the current behavior adds more confusion, where the hyperlink takes user to a new page displaying an error message.
2. On the "About the Extension" popup window, add a hyperlink to point to the extension link, where you have a lot more details about the extension.
2a. {Bug} The [x] to close the popup window doesn't work. Nothing happens when it's clicked. User has to click Ok to close the window.
3. Add right click options on the Add-On icon, which would give user two menu options. (Capture All Tabs in All Windows) and (Capture All Tabs in Current Window). See OneTab right click menu option as example.
4. Add option to capture tabs, and then close them. This would be similar to what OneTab does.
5. Allow scheduled backup sessions to be edited.
6. Add option for an On-Close Session Backup. This would capture all the tabs when user closes the browser. This can be displayed on a new tab, only save the last 10 or 20 closed session backups. If programming automatic capture on close can not be done, then add a button to the right click menu to capture session and close the browser.
7. Add help option. It could just take user to the Session Boss extension link for now, until a page with more help can be developed.
8. Add accumulative backup option.
I was expecting that the hourly backup would be a consolidation of the last 4 15-min backups. But it's a regular backup performed on hourly interval. I recommend adding option that would let user choose to have accumulative backups starting at the hourly interval. With accumulative option enabled, the hourly backup would be an accumulation of the last 4 15-min backups, and the daily backup would be an accumulation of all the hourly backups for that day. Monthly would be accumulation of the days. Have a yearly backup with a 12 month accumulation. The accumulative backup can be displayed in a tree view, where a year node has 12 (monthly) children, and each child has 28-31 (daily) children, and each child would have 24 (hourly) children. At the hourly node, it would display 4 (15-min) children, and those children would display exactly what the 15-min backup displays in the current UI.