Session Manager is wonderful, but not perfect. Rated 5 out of 5 stars
Session Manager is necessary. Version 6.02 of Firefox is very unstable, far more unstable than version 3.6.20. Firefox crashes often when there are 100 tabs open, a situation that is common when doing research.
Session Manager is wonderful, but not perfect. Often it does not restore windows to the size at which they were saved. When restoring, apparently all tabs are re-loaded from the internet, instead of from the cache.
See the "reload all restored windows" options
It's misnamed since it's function has changed, but unchecking the "reload all restored windows" option in the General Saving & Restoring options will cause Session Manager to restore sessions from the cache (if possible) instead of from the Internet.