Since Firefox and Chrome both implemented cache partitioning, extensions can't download images from cache anymore.

Firefox:

• cache partitioning causes web page saving extensions to waste more time and data
• Allow extensions to specify request context in the browser.downloads.download API
• Enable extensions to send network requests (fetch) with a specific cookieStoreId (container tab context)

Chrome:

• cache partitioning causes web page saving extensions to waste more time and data - which is marked as wontfix 😕

  I don't believe allowing extensions to look in any cache partition or even in just the current page's cache partition would be feasible since that would allow malicious extensions to be able to infer if a resource is cached or not by the user.

Test suite

https://github.com/eight04/webext-hit-the-cache

Firefox workaround

You can turn off cache partitioning by setting privacy.partition.network_state to false in about:config.

They removed this config: 1954293 - Remove privacy.partition.network_state

We added a new option to intercept image from webRequest API in Image Picka 0.20.0.

Chrome workaround

Turn off this feature via the CLI option: --disable-features=SplitCacheByNetworkIsolationKey.