Conflict with other extension

If the Error button in the Tab Mix Plus Options is visible, some of your extensions are incompatible with Tab Mix Plus. It is recommended you disable or uninstall the extensions in the list. Click the "Show List…" button to do that.

Conflicts have been reported with Ubuntu Firefox Modifications extension, so if you use Ubuntu, disable that one (you can read more here).

Conflicts with BarTab, Colorful Tabs and Tree Style Tab have been reported. Since all these extensions manipulate tabs, shouldn't be surprising. Don't panic, these conflicts are usually solved rather quickly, especially if good information is given (like errors in console, screenshots, steps to recreate, etc.). Feel free to post and provide as much info as possible.