Follow these instructions to locate your profile directory.

Zotero itself doesn't tell Firefox how to handle a file—it simply checks if a file can be handled within Firefox (natively or via a plugin) and then either sends it to Firefox if so or otherwise launches it via the OS. Problems in mimeTypes.rdf can prevent this from working properly, however.

You can find your Zotero Standalone profile directory in the following location:

/Users/<username>/Library/Application Support/Zotero/Profiles/<randomstring>

C:\Users\<User Name>\AppData\Roaming\Zotero\Profiles\<randomstring>

C:\Documents and Settings\<username>\Application Data\Zotero\Profiles\<randomstring>

~/.zotero/Profiles/<randomstring>