Since Firefox version 3, it's done aggressive caching of visited pages, and
does not, by default, check if the cached content is still valid when
reloading or revisiting a page. By default, Firefox seems to not re-contact
the web server at all. Compare with eg, Epiphany and Chromium, which appear
to always check, and get back a 304 when the page is unchanged.
This header makes Firefox do the right thing, at least for html files. It
still over-caches if css, javascript, images, etc, are changed.
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