One of two possible places Adobe Illustrator Application folder under presets sysmbols or
Users/Username/Library/Application Support/Adobe/US Eng/Symbols
if not try this from the symbols panel flyout select save symbols library and choose a location such as freehand to AI Symbols.
But estentially the symbols are in the document so if you wanted to import tham into another document all you really have to is select the document when loding a symbol library but for your purposes it would be best to have it with a name referring to the symbols themselves and in a particular folder named for those symbols.
I'm noticing other flaky behavior with FreeHand converted files. My intention is to open those files, copy the contents, and then paste them into new AI documents. That seems to resolve most of the issues.