Experimente esta dica nas páginas de documentação da comunidade do mozilla :
Restoring default search engines
Default search engines that come
pre-installed in Firefox currently
include Google, Yahoo, Amazon.com,
Answers.com, Creative Commons, eBay,
and in Firefox 3, Wikipedia. If you
have removed any of the default search
engines. you can restore them using
the "Manage Search Engines "Restore
Defaults" button. This will not remove
any added search engines. (If the
"Restore Defaults" button is disabled
or "grayed-out", as shown above, that
means that all of the default search
engines are still available).
Another way you can restore the
default set of search engines is to
use the Firefox Safe Mode option,
Restore default search engines. This
will add back all of the default
search engines if some of them are
missing. This will not remove any
added search engines.
Você pode invocar o firefox no modo de segurança a partir da linha de comando:
firefox -safe-mode