As a very happy long-term Session Manager user, I salute your efforts here! Simple and effective. This should help with our Session Manager withdrawal symptoms.

A few thoughts for further refinement:
1) Logo barely visible on gray ribbon. A multi-color icon would help visibility for diverse color needs. Look too at Firefox's closed window/tab icon; white but outlined in black, thus visible on any background really
2) A third save scheme (daily) would be useful, perhaps at first use each day, i.e. 12:01am or whenever Firefox is next open
3) The list of saved sessions might be easier to digest as single line entries; you might need to abbreviate some data, such as save type (Regular (or Interval), Daily, Closed, and User)

Thanks for launching this!