Mozilla suite is more mature than the 'birds (Firefox used to be Firebird, and the mail client is Thunderbird), but is being wound down in favour of the standalone parts.
Firefox is now hitting 1.0 Preview edition, while it looks like the Suite will be carried forward to a 2.0 "final" release and maintainence only after that - but with no real development even up to that point.
It's a switch - the standalones started as an offshoot of the suite, now the suite is being pushed to the back.
If you want the "Composer" as a standalone, you have to go to NVU, and maybe some other elements of the suite have not actually got a counterpart in the standalones - not everyone is happy with the switch, or with the way certain improvements to the suite have been "won'tfixed" as they pile forward with the standalones.