There might be, but it may not be easy. The proxy settings that I use are stored in the prefs.js files. Editing this file using vbscript should be relatively easy. However, this is under my user profile and is accessible by me only. Installing Firefox for a multiple user environment and whether it stores the proxy in the same file, I don't know. Also, under my user profile, the path is semi randomly generated as the path is
C:\Documents and Settings\profile\Application Data\Mozilla\Firefox\Profiles\ye1vmizp.default
It's that ye1vmizp.default part that is random. How you'd script for a folder name that's randomly generated, I'm not sure, and again, whether it's the same in a multiple user environment, I'm unsure.