> I guess the one built into Firefox is just downloaded from there and cached? If so why would you want that over the other?

If your extension is 10kB of Javascript and you typically update it once or twice a year to tweak things, it's crazy that now the total extension size is over twenty times bigger and you need updates every month or so at least because otherwise things might not work for some users.

If your extension wraps, say, the New York transit map, or Wikipedia's list of English monarchs then fine, there's no reason Firefox would know those, you need to ship or fetch the data. But the PSL is necessarily built-in to Firefox, they do have the data, you just can't access their copy.