Type about:addons<enter> in the address bar to open your Add-ons Manager. Hot key; <Control>(Mac:<Command>)<Shift> A)

In the Add-ons Manager, on the left, select Extensions. Remove Yahoo Anything. Now do the same in the computers Programs Folder.

Windows: Start > Control Panel > Uninstall Programs. Mac: Open the "Applications" folder. Linux: Check your user manual.

After the problem is fixed, Bookmark and use this; Download the Mozilla Search Reset {web link} This add-on is very simple: on installation, it backs up and then resets your search preferences and home page to their default values, and then uninstalls itself. This affects the search bar, URL bar searches, and the home page.

Firefox is designed to let you choose your preferred search engine and home page and remember those settings permanently. The only scenario in which Firefox changes your search engine back to Yahoo (for North American users) and your home page (back to the built-in home page) is if you use the Refresh feature to clear your add-ons and settings.

But as Fred mentions, bad add-ons very commonly take over your search engine and home page settings so it's worth taking very close look at those to root out troublesome ones. Especially get rid of anything named SearchMe, or from Conduit or Spigot.

Finally, please be aware that some external utility software may roll back your browser settings changes. Such programs include Advanced SystemCare among others.