You can modify Firefox with and extension, I personally would not want that at all, before dismissing a tab I make sure there is nothing in the tab in a backward direction, and the and would consider looking a anything but a blank page for a new tab as a distracting encumbrance but you can do what you ask for with the following:

• NewTabURL :: Add-ons for Firefox
  https://addons.mozilla.org/firefox/addon/newtaburl/

I have three pages on my own on tabs

• Tabbed Browsing in Firefox
  http://dmcritchie.mvps.org/firefox/tabs.htm
• Tabs configuration, extracted from About:config entries - MozillaZine Knowledge Base
  http://dmcritchie.mvps.org/firefox/tabs_config.htm
• 001 Tab Capacity and other Tests
  http://dmcritchie.mvps.org/firefox/tab_capacity/001_with_underscore.htm

I also like to change the styling on tabs and other chrome aspects of Firefox

• Styles & Extensions that I use, see picture (Red borders -- active tab, Blue borders -- tab has been read, Green borders -- tab has not been read, Magenta borders -- tab is being downloaded)

I have looked for extensions/add-ons, but I cant find any that are compatible with Firefox 5.0, only with 4.0 I dont want a blank page when opening a new tab, I want it to open google

I know you looked that is why I provided the link to an extension (NewTabURL) that does what you asked for, good for Firefox 1.5 through Firefox 6.* and you're on 5.0 .