There's an add-on for that.

Usually I suggest this one, but it's not yet updated for Firefox 5:

New Tab Homepage :: Add-ons for Firefox

So you could try this one:

NewTabURL :: Add-ons for Firefox

The add on that you suggested didn't seem to work. I used the following add on and I am now able to set my homepage to open with with each new tab.

howeverhttps://addons.mozilla.org/en-US/firefox/addon/tab-utilities/

Thanks for answering though - I thought I just couldn't find the function within this version of Firefox