Hi, as far as I know, zstd compression should still be working, at least for HTTPS. Is this http or htttps website?

"zstd" appears in various places in the preferences editor, about:config. I'm not sure which is relevant to your issue. (Configuration Editor for Firefox)

(1) Network transmissions (since Fx126)

• network.http.accept-encoding = gzip, deflate
• network.http.accept-encoding.secure = gzip, deflate, br, zstd

(2) Content? (since Fx138)

• dom.compression_streams.zstd.enabled = false

(3) Certificates (since Fx143?)

• network.http.http3.enable_certificate_compression_zstd = true

website https Browser response:

Content Encoding Error The page you are trying to view cannot be shown because it uses an invalid or unsupported form of compression. Please contact the website owners to inform them of this problem.

other browsers return the correct code Sorry, I don't want to post the link because it leads to the site administration page.

As a starting point, could you experiment with the setting:

(1) In a new tab, type or paste about:config in the address bar and press Enter/Return. Click the button accepting the risk.

More info on about:config: Configuration Editor for Firefox. The moderators would like us to remind you that changes made through this back door aren't fully supported and aren't guaranteed to continue working in the future.

(2) In the search box in the page, type or paste network.http.accept-encoding.secure and pause while the list is filtered

(3) If the preference value does NOT have "zstd" (it isn't is "gzip, deflate, br, zstd"), double-click and add zstd to the list. Does the page load?

(4) On the other hand, if the preference value HAS "zstd", double-click to shorten the value to "gzip, deflate, br" to roll back support. Does the page load? If so, perhaps the implementations of zstd between Firefox and the server do not match for some reason.