http://www.google.com/firefox

are there any other custom firefox start pages?
is there a version for yahoo?

As far as I know there isn't a Yahoo version.

Who needs one?

i don't think there are custom pages in yahoo search

but for google you have
www.google.com/ie
www.google.com/bsd
www.google.com/microsoft.html
www.google.com/intl/xx-hacker/
www.google.com/mac.html
www.google.com/firefox
...

1. The logo on the BSD page would look great with a "Don't Be Evil" caption.

2. It's deceptive, on the IE page, having the link to download the toolbar looking like a search box. I bet quite a few people click on that by mistake.

you can make you custom google page:

   http://www.google.com/custom?cof=%22L:http://lh6.google.com/bilal.douas/R7NKc7SjgWI/AAAAAAAAAFU/J_M0KO0n_nI/pacoocle.jpg?imgmax=400%22

Does this count as one for Google?
http://www.google.com/ig/dell

Mrrix32, I believe that is just a specialized iGoogle page for partners.