Firefox does not send If-Modified-Since header

I'm developing a web app and I'm trying to cache the result of a query in the browser.

I send the Last-Modified header, and it works as expected on Chrome. When I repeat the request, the server gets the If-Modified-Since header and I can return the status 304 (not modified).

However Firefox does not sends If-Modified-Since. But if I run the same app on localhost, it works. Any idea please?