First is Tab Mix Plus. It adds plenty of setup options for Firefox tabs, but the most important for me is “protect tab” option. It lets you select which tabs are protected from being accidentaly closed, eg when you close multiple tabs at once. This is useful a tab with Google Docs or Gmail for example.

Tab Mix Plus also lets you have the tabs in multiple rows. This is very useful for people like me who use to have plenty of tabs opened at the time. Another option is the second plugin called Tree Style Tab.

Tree Style Tab moves your tabs to sidebar and even lets you have related tabs in a tree-like hierarchy.

See both plugins in action in this picture:

Firefox tabs plugins