testing extensions on FirefoxPortable

I'm an extension developer and as I use several PCs I would really interested in being able to develop my extensions on a USB drive (which I otherwise use constantly). Right now I have the development version of my extensions on the USB drive and use a local installation of Firefox to test them.
I've been trying to install the extensions on FirefoxPortable (and previously on PortableFirefox) using the same method I use for testing: just create a file on the extensions folder, with the content being the path to the extension folder. That doesn't work on FirefoxPortable. Not with absolute paths, not with relative paths.
Do you know why? Is there something missing in the compilation?
Is there someway to fix it?

I can always automate the process of creating a XPI file and installing it, but it's just not flexible enough.

Thanks.
tnarik