It looks obvious

Flock is a browser with agenda and it follow its agenda with several features. However Flock’s agenda limits your choices. For example: the nice top bar interface which open a stream of photos; The API allow you to choose account in one, or both, of the photo services: Flickr and Photobucket . However if your account is with different service, and there are many of those – some are pretty famous, you are out of lack. The same example applies to the favorites API: if your choice of service is RawSugar – you are out of luck again.

I do not claim that Flock deliberately preclude some services and favor others, and I’m sure that as time pass Flock will have API to more services. However it will never have all of them.

One of the most appealing feature in firefox is its lack of special features. The basic browser is almost naked. But this lack of feature isn’t a weakness, it is one of its strength, together with the ability to easily add extensions. A very light browser become after a short period of time to extremely personalized browser.

One might point to the fact that Flock can be personalized with very similar, often the same, extensions. Nevertheless this extensions are a layers on a browser that carry features  that one might not need.

Ultimately users will make a decision for a browser that best suit them. Flock positioned itself for a niche, Firefox in the other hand is positioned to compete for on the masses.