To solve us.search.yahoo tab hijack problem, you have to follow this steps:

1. In the Location bar, type about:config and press Enter. The about:config “This might void your warranty!” warning page may appear.
2. Click I’ll be careful, I promise! to continue to the about:config page.
3. copy from here the next : browser.newtab.url , and paste it in the search area
4. right-click on it, select modify and write whatever you want (for example google.com)
5. click ok, exit firefox and restart it.

see for more info : New Tab Page – show, hide and customize top sites