I somehow had a problem described in this post, that is, when I open a new tab in Firefox and the new tab always opens a Yahoo Search page regardless the "When Firefox Starts" and "Home Page" settings in the "Options" menu. I blame myself for not careful when installing software and browsing the web. I am looking for a fix.
Followed the post cited above, I did not find any hidden add-ons. The solution did not work for me although it worked for some according to the comments left on the post.
I eventually have a fix for this problem.
When I open "about:config" and search the word "Yahoo", I find that the setting "browser.tab.newurl" is pointing to a Yahoo search page. After I set the value of the setting as blank, the problem goes away.
By the way, the Firefox discussion forum have some good discussion on related issues.