No, the official versions of Firefox don't come with Yahoo, but there is a Yahoo! edition of Firefox that is available from non-official websites.

Open Help > About Firefox and see what is in that small window (official version is shown below). You'll probably see something about Yahoo and a version 1.8. Uninstall that version of Firefox from the Control Panel and install an official version of Firefox 4.0.1 from an authorized download mirror.
http://www.mozilla.com/en-US/firefox/

I tried the posted suggestion and it did not work. I do not want Yahoo in my Menu bar.

To elaborate my previous post I tried the posted suggestion and when I start Firefox for 2 seconds there is no Yahoo but then it pops back into Menu bar. What next?

I agree with bobsscgirl...I would like to know how a user can control the content of the 'Menu Bar'.

I downloaded the official version from Mozilla Firefox website and it has Yahoo as one of the Menu Bar items.

Just found this: [https://addons.mozilla.org/en-us/fire.../menu-editor/]

Yahoo! is on the Menu bar in my official release of Firefox too. I want it gone. I downloaded the Menu Editor add-on, and haven't figured out how to get Yahoo! off the toolbar.

I finally was able to rid myself of the Yahoo add on. Go to tools on menu bar, click add on, then find extensions on right and click, then remove yahoo tool bar and restart. Gone!

I had tried the normal ways of removing these pest such as using the Control Panel to Remove Yahoo (it wasn't there) and using Tools->Options->Extensions or Add-Ons and the Yahoo stuff didn't appear in any of them. I could hide the toolbar by right-clicking but I could not remove it, nor could I remove the Yahoo Menu.

  This is what I did that  DID remove the parasite.   I went to Tools->Options->General->Manage Add-Ons->Recent Updates (on the bottom) and it was there.  It didn't appear anywhere else.  I then hit the remove button.  Then clicked the Restart Now tab at the top.
  I don't know what I would have done if it wasn't still in the recent add-ons because I have been unable to find it in any of the other "normal" places for removing these pesky things.