I upgraded to version 5 on my laptop running vista and the search function does not work:

Which search are you referring to and what does it do instead.

I am referring to the search line on the firefox opening page. When I enter something and click on "search" nothing happens.

When the about:home searchContainer is disabled after an update.

https://bugzilla.mozilla.org/show_bug.cgi?id=637639
if dom storage is disabled when the profile is upgraded, about:home remains broken even after dom storage is re-enabled

https://bugzilla.mozilla.org/show_bug.cgi?id=637561
about:home does not check for disabled dom storage (null localStorage)

Reset this pref after enabling dom storage
Type about:config in the URL bar and hit Enter.
If you see the warning, you can confirm that you want to access that page.
Pref = browser.startup.homepage_override.buildID
Double-click that pref to reset it.
see - https://bugzilla.mozilla.org/show_bug.cgi?id=637639#c5