FuelPHP1.6が出たので、製作中のWEBアプリのフレームワークをFuelPHP1.5→1.6に更新してみた。
すると、

Composer is not installed. Please run "php composer.phar update" in the root to install Composer

のエラーが発生する。

FuelPHP1.6のChangelog に載っていた通り。

This release officially introduces Composer to FuelPHP. 
You will have to install it, and run a 'php composer.phar update' to pull in any required packages. 
Without this step, 1.6 WILL NOT WORK!!!

サーバーにログインして、コンソールから、php composer.phar updateを実行。

# php composer.phar update
Loading composer repositories with package information
Updating dependencies (including require-dev)
  - Installing fuelphp/upload (2.0)
    Downloading: 100%

  - Installing psr/log (1.0.0)
    Downloading: 100%

  - Installing monolog/monolog (dev-master 1eed379)
    Cloning 1eed379a3b7c31f698e49fa7a822a536e94c31f5

monolog/monolog suggests installing mlehner/gelf-php (Allow sending log messages to a GrayLog2 server)
monolog/monolog suggests installing ext-amqp (Allow sending log messages to an AMQP server (1.0+ required))
monolog/monolog suggests installing ext-mongo (Allow sending log messages to a MongoDB server)
monolog/monolog suggests installing doctrine/couchdb (Allow sending log messages to a CouchDB server)
monolog/monolog suggests installing raven/raven (Allow sending log messages to a Sentry server)
Writing lock file
Generating autoload files

無事FuelPHPの画面が出ました。
FuelPHP1.6