Here's what the Firefox site says about that plugin (compatibility issues)

http://support.mozilla.com/en-US/forum/1/571763

I'm not sure if you will lose any files/folders/profiles/etc for both versions if you uninstall 3.5. Both versions store certain folders in other places besides the regular programs folder so they may be sharing
certain files. If no one else here has ever tried that you may want to ask at the firefox forums linked to above.