Sometimes your Tab Mix Plus options fail to be saved after you change them. For example, you may change a setting in the Tab Mix Plus Options window only to have it set back to its previous value when you reopen the Options window or restart Firefox.
Read this article from Mozilla support for possible fix.
If you still have trouble keeping preferences from getting reset, try to temporarily disable your other extensions one at a time (restart Firefox after each extension you disable) until the problem disappears.
Report to our forum if after you disable specific extension the problem disappeared.