Google recently made some changes to the Google search site with the addition of Instant search and that made the setting of the number of results in the Google settings to stop working.
If Instant search is enabled on the
Google Search settings page (link at the top right) then you only get 10 results.
You have to disable instant search and reload the result page and then set the number of results to your preferred choice.
It may not work if you make both changes at the same time.
There is a link directly to the right of the Google search field on the Google results page.
That is a button with a drop down list: Instant is On/Off.
Google Advanced Search only displays 10 Results per page in Firefox 3.6.10 | Firefox Support Forum | Firefox Support