First question, is the Standalone library the correct one? If so, it would be easiest to update to Zotero 5 and the new Firefox connector (as of Zotero 5, the full Zotero plugin inside Firefox is discontinued, so this would be the fastest way to resolve the issue here).
Otherwise, you can find the Zotero data directory in the Advanced pane of the Zotero preferences. Set the two programs to point to the same file path. Sync both programs to your account first to ensure that all changes in both are reflected in your online account.
Upgrade Storage
Otherwise, you can find the Zotero data directory in the Advanced pane of the Zotero preferences. Set the two programs to point to the same file path. Sync both programs to your account first to ensure that all changes in both are reflected in your online account.