Un-install the Ask toolbar, it is known to prevent Tabs from opening.

Didn't even know I had one, so I told it to display then went to the right hand end where a drop down menu offered "uninstall" and fired that , and again, and again, and again, the sticky little booger grinned back, but would not leave. Reminded me of early Windows days when Win would die and tell you, "I just lost all your work" "OK?" Going to look for a big hammer, it used to make me feel good to have the last FINAL word.

Yee-haaa, I shut down Firefox while I went for the hammer - and the uninstaller fired and offered to remove it. Thank you, thank you, thank you, reopening Firefox after removal, I held my breath and clicked "+" , then exhaled with a shout of glee as a new tab appeared, again and again and again. Yes Sirree you fixed it goo-o-ood. Dump the ASK bar.