At the time I write this, this is a known issue:
http://garyr.net/tmp/forum/viewtopic.php?t=15821&p=55432
The only workable solution I've found is to disable session management in Tab Mix Plus*, enable the Firefox setting to only load tabs on demand (under tools -> options -> tabs -> "Don't load tabs until selected"), and, if you still want similar (probably better) Session Management functionality, install the Session Manager extension --
https://addons.mozilla.org/en-US/firefox/addon/session-manager/ --
This extension has an equivalent setting under Tools -> Session Manager -> Session Manager -> General -> Saving and Restoring -> "Restore Tabs on Demand."
*As I've just installed the Session Manager extension, which modifies Tab Mix Plus' options interface, I don't recall exactly where to do that.