Since I test Mozilla.com What's New and First Run pages, I thought it'd be useful to write down when which appears when.
I've put it up at our web testing wiki, and here it is, below:
* Freshly installed Firefox, you get:
o First Run (left tab focused/active)
o Firefox start page on Google.com (right tab)
* 3.0.x -> 3.5.x, you get:
o First Run (left tab)
o What's New (right tab focused/active)
* 3.0.x-1 (next-to-latest version) -> 3.0.x (latest version)
o Firefox start page on Google.com (left tab)
o What's New page (right focused/active)