UPDATE I just read that
extensions.ini is not important at all and is automatically generated. So I guess I am wrong here, also given the
[HKEY_LOCAL_MACHINE\SOFTWARE\MozillaPlugins] key as mentioned by @routeNpingme. I'll remove this later today; just leaving it here to ensure others don't waste their time on the same thing.
Given the fact that Firefox allows for using different profiles, each with their own extensions, I never thought that it would use the registry as well. (Firefox being cross-platform makes me even wonder more about that, especially as Firefox already has it own kind of cross-platform mechanism of storing settings.)
So unless you're sure it's in the registry, you might want to peek into the Files and folders in the profile, especially
Lists folders of installed extensions and themes. The file is automatically generated by nsExtensionManager and is used by low-level code to detect chrome packages and XPCOM components provided by installed addons. Can be deleted to resolve various issues.
(See command line arguments to start Firefox with another profile.)