On Firefox, if you have an unbelievably large number of entries in your History, Bookmarks, and/or Downloads (your Firefox "Library" — chrome://browser/content/

On Firefox, if you have an unbelievably large number of entries in your History, Bookmarks, and/or Downloads (your Firefox "Library" — chrome://browser/content/places/places.xhtml), you could theoretically become limited by insert/update performance in Firefox's embedded sqlite, I believe.