You're forgetting the super-geek demographic. The kind that don't want Linux-Lite that Android offers, but want the whole shebang you get with Maemo. That being said I own an N900 and it is by far the best phone I've ever had. I've played with friend's iPhones and Android phones before and they never appealed to me. The devices felt cheap... not hardware cheap.... software cheap. The limitations of which could be felt within 5 minutes of using the device... heck you can't even multi task on the iphone.
One of the first things I did when I got my N900 was first install gainroot, takes like minute from the app manager, then installed Ruby 1.8.7, then found some Ruby tutorial pages and began copy/pasting Ruby code snippets into Conboy notes that I keep synced up. Then I make files using PyGTKEditor and execute them from terminal. So now whenever I'm stuck in a boring situation... aka the bus, lame party, work gatherings... I pull out my phone and continue learning Ruby so I don't feel like I'm wasting my time.
Ohh I also remapped my arrow keys to do Home/End/PageUp/PageDown when I hold the function keys, and remapped the Pound and Euro symbols to < and >.
I don't know if any other phone would have let me do all this stuff... and it's just a fraction of the fun of having a full blown OS... but I feel that if I had an Android I'd be hitting walls of frustration.
The reason FireFox went with the N900 is because Nokia spent alot of time working on it for the Maemo internal browser. The FireFox mobile project probably got alot of free code from Nokia which made writing FireFox for the N900 the easiest task to tackle. Which I still prefer over the FireFox simply because it's much more integrated into the phone, and I've yet to had a website not work in it.