Unknown, testing by creating a new User in Accounts in System Preferences, logging in as the new user and testing...
will indicate if the setting (plist) is in your Users's Library or the OSX Library.
Maybe this - instructions for FMP 5.5
3.1 Quit the FileMaker Pro application if it is currently running.
3.2 In the "FileMaker Pro 5/FileMaker Extensions" folder rename the "Web Companion" file to "Web Companion old" and move it out of the "FileMaker Extensions" folder.
3.3 Move the new, downloaded "Web Companion" file into the "FileMaker Pro 5/FileMaker Extensions" folder.
3.4 Launch FileMaker Pro.
3.5 After verifying your web solution(s) function properly, delete the "Web Companion old" file.