The “unified partition” update (or storage upgrade) changes the partition layout of the FP1, which came with two partitions out-of-the-box. One partition (1 GB) was named “internal” partition, the other “phone” partition. Users were sometimes confused about this, since the standard installation procedure by android installed apps in the internal partition which thus was quickly out of space.
An article about the update to a unified partition can be found here. (If you follow this article, be sure to do a proper, full backup of your phone - the out-of-the-box “Backup and restore” app is likely not sufficient for your needs.)