I recently had my Firefox profile corrupted and I had to restore default settings with this procedure.
Previously I had a "new tab" button on my toolbar, and now it doesn't seem to be available anymore. Please note that I'm not talking about the green "plus" icon in the end of the tab bar, I had a "new tab" button on the main toolbar and now I'm used to go there if I want to open a new one.
I use "Tab Mix Plus" but if I'm not mistaking that button was a default Firefox feature, not from this add-on.