>>1061400
Last time, it was because they switched add-on mechanisms to become more like Chrome, forcing a full rewrite of many add-ons, such as Tree Style Tabs. At the time, the add-on stored tabs differently from the way stock tabs were stored, so a Firefox update that killed it also deleted its settings, eliminating all tabs from prior sessions.
This is no longer the case, so if (when) SJWzilla decides to make the add-on mechanism even more restrictive, tabs from prior sessions will still be accessible using Firefox's built-in tab and session managers (unless those are removed too).