This is a very good post Paul. A couple of random thoughts in the interest of time:

1. average products often succeed in spite of themselves – the fact that twitter is bad in so many ways is evidence of this, eg, the ubiquitous “fail whale”

2. Paul Buchheit is unbelievably clever (read his chapter in “Founders” – world class – can lend it to you next week), but that doesnt necessarily prescribe success (this is obvious, but is worth mentioning because we should pay more attention to FF because of Paul, IMO)

3. FF has chosen the threaded model (like gmail and RFC 2822) and I agree it is intuitive but again it isnt what people have been inculcated to use. Just because it makes more sense doesnt mean it will be market successful (obviously) – betamax, et. al

4. The “use them in tandem” bit: FF is meant to be an aggregator – meaning one stop shop for all my social media stuff? This alone (lack of user base) will likely surely see it fail.

5. Simplicity. Twitter is widely successful is because it is VERY simple. My mother (62) is on it. FF at first glance appears to have feature bloat. Tries to do too much (yes, changes have negated much of this but not enough). Accordingly, it appeals to a small, seemingly cerebral type of person like yourself, but unlike me and the rest of the masses.

The key argument in all of this that would likely sway me is the Search functionality that you brought up. Clearly Buchheit has brought his experience to bear here. BUT, this is something Twitter can fix very easily through being acquired by google, facebook (great search now) or even microsoft (great search, its problem is its brand). Also through an acquisition perhaps – there are many semantic search engines that are really good.

So I am still unconvinced by FF. Sorry.