The Nokia N900 has about 100MB of free space in the root file system partition. It doesn’t take installing many applications before it fills up. Nokia recommends installing applications to the /opt folder, which has about 1GB of space.
Fennec installs itself to the root file system in the /usr folder. Although this is fine for the N810, it would never work for the N900. We had originally planned to move the install location for Beta 4, but we found a bug that kept Fennec from launching from the /opt folder. The bug has been fixed and Fennec now installs to /opt in nightly versions.
A nice side effect of /opt installation: startup time is ~1 second faster!