My home directory is on a separate partition. I am on 10.10 from 10.04 via upgrade. How can I perform a clean install of 10.10 while keeping this partition? Will it perform as well as a clean install? What will be lost and what will be kept?
When you fresh install Ubuntu you have to manually manage partitions. You can decide where to mount / (root) and format that partition, then you select where to mount your home and choose to not format that partition (you don't check the format option). In this way you don't lose anything even if some configuration folder (.mozilla for example) may change. I suggest to backup Home folder before, in case of errors you can remedy.
Here there's a good tutorial.
this is possible.
When you arrive at the hardrive information section of the installer, ensure you select the option to configure partitions manually.
Ensure the partition that contains your home folder is not marked for formatting. You can then manually set its mount point as
The rest of the system will be installed as normal on the remaining partition.
In theory you can just install the new ubuntu and make partitions manually during the installation process, choosing to keep you old home partition (i.e. without formatting it) linked to the /home mount point.
The problem is that in your home all the configuration files for gnome, firefox and a lot of other big software you probably had in your old system, are kept. So installing a brand new system with such configuration files already existing may become a problem if some software has been changed a lot since your old ubuntu was released.
So in practice I would suggest to make a copy of the data that's in your home, install the new system keeping your home as I said, and if some serious problem with some software arises you can delete the user (along with its home directory) and re-create it.