Why does JavaScript's LocalStorage behave like SessionStorage?

I added LocalStorage to a web page. It seemed like it was working well: when I reload the page, it found and used the existing data. Close the window, when another browser window is still open, then load the URL again: this also works fine, it finds the previous data.

But when I closed all browser windows, and restarted, the data was gone. In other words it appears to just be storing the data per-session, not indefinitely as should be happening. This was happening with both Firefox and Chrome!

It turns out it was configuration, in each browser! See below for the solutions for each. What is frustrating is that neither browser gave any error message; I'm not even sure there is a way to query that the storage is going to end up session-only. I didn't even expect LocalStorage to have the same privacy policy as cookies: cookies get sent back to the server, whereas LocalStorage does not.

Anyway, to the fixes:

Firefox

In Firefox Preferences, Privacy tab, I had both "Accept cookies from sites" and accept "third-party cookies" checked. But under 3rd party cookies I had "Keep Until I Close Firefox". When I changed it to "Keep until they expire" then LocalStorage started working.

This is pretty strange, as it sounds like LocalStorage is considered a 3rd party cookie.

By the way, in about:config, there is also dom.storage.enabled. I had this set to true. If it was false that would be another reason it would not work (though I think then it would not work when just pressing reload either).

Chrome

In Chrome, settings, advanced settings, content settings: under cookies I had "Keep local data only until I quit browser"
One solution is to the change that to "Allow local data to be set (recommended)". The alternative is to add an exception of "allow" for the domain name in question.