Can the Developer version be installed over a Release version?

You should be able to upgrade FirefoxPortable stable to FirefoxPortableDeveloper. You will need to manually adjust the install path as the installer will default to PATHSELECTED\FirefoxPortableDeveloper. I would recommend upgrading to the latest stable version first and backing up.

Ideally, you should first upgrade FirefoxPortable to the latest release. Then in Explorer, copy the FirefoxPortable directory and paste it in the same directory using either the menus or CTRL-C and then CTRL-V. Rename the newly created FirefoxPortable - Copy directory to FirefoxPortableDeveloper. Then upgrade the FirefoxPortableDeveloper copy to Firefox Portable Developer Edition using the installer. This will keep your original copy intact at the original location and give you a new copy of the developer version in its default location making it easier to upgrade manually or automatically with the PA.c Platform (the preferred method).