The entries would still be datestamped and as firefox clears up the history the older ones would be deleted.
In my other comment I linked something that said firefox just starts tidying up as performance starts to degrade, rather than a fixed limit of history entries.
Therefore, if your history is more or less full, and you just import a heap more history, firefox is just going to “tidy up” everything you just imported surely.
The entries would still be datestamped and as firefox clears up the history the older ones would be deleted.
In my other comment I linked something that said firefox just starts tidying up as performance starts to degrade, rather than a fixed limit of history entries.
Therefore, if your history is more or less full, and you just import a heap more history, firefox is just going to “tidy up” everything you just imported surely.