Sounds like you want the New Tab Homage extension. https://addons.mozilla.org/en-US/firefox/addon/777/

For more info about extensions, see Use extensions to add features to Firefox

Just as an aside: this add-on doesn't work for 4.0b4 yet.

In general, I really can't believe that Firefox, now, what, 5 years old, still doesn't have this as a built-in feature. I mean, how hard could it be to put a little checkbox next to ones home page preference to indicate that this should be used also when opening a new tab! This has been requested by users ever since the beginning! It shouldn't have to be an add-on.

Ah...Excellent....I've been trying to do this for some time and thought it must be something I was doing wrong. After all, I'd set my Homepage; so I just expected any new tab to automatically open at my Homepage! Thanks to the author of the extension. P.

Found the answer in another thread discussing the same problem. Hit Ctrl and click the Home icon at the same time. Works great!