Why does Firefox ignore cache headers and revalidate on refresh?

Question

I have some image resources that are immutable and can be cached forever. Chrome seems to respect my response headers, and does not re-validate the resources:

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Answer 1

Just to be more verbose let me elaborate on ETags. Technically, Firefox is handling it correctly. When an Etag is present, the client or intermediary caches need to perform a GET or HEAD ensuring the content being served hasn't changed.

The If-None-Match header tells the server what version or versions it has. If it has not changed, the server/proxy returns a 304.

You can modify the revalidation behavior by making it a weak ETag. That is done by including W/ in front of the ETags value, i.e. ETag: /W #########. This can be handled on your server or if you can't control it by rewrite rules on your CDN.

The possible workaround for Firefox is adding the Cache-Control Immutable options. Cache-Control: Immutable. Without looking at the code in Firefox caching heuristics I can't say for sure but it should be easy to test.

Other servers will ignore the header if they don't support it.

Etags also don't work with byte-range requests. So if you don't have a specific reason for using them consider turning them off.