Add support for zstd compression

[siewca]

There is old bug in bugzilla for it: https://bugzilla.mozilla.org/show_bug.cgi?id=1301878
I have implemented some version of HTTP/SCGI service for serve static content with compression (gzip/br/zstd) with (fast) compression on the fly and worker thread for (best size) compression those files - it could be used for testing performance during 1st stage of implementation.
Code are here https://opensource.krypto-it.pl/kit-static-scgi/ (GPL2) and works e.g. with nginx.
Now zstd is supported e.g. by curl, so maybe is good option to add it to firefox.

[Jon]

Thanks for submitting an idea to the Mozilla Connect community! Your idea is now open to votes (aka kudos) and comments.

[rockdaboot]

Android, Opera and Chrome started supporting zstd: https://caniuse.com/zstd

[siewca]

It's possible to use static content on other domain (e.g. www.example.com) then main site (example.com) and use TLS with compression for static content without send cookies to subdomain. Normal paid TLS certificates for 1 domain are for 1 domain + www., so according to www.breachattack.com sites can use compression for static content without session data and normal works without compression for dynamic content.

PS.: My link upgrades: https://opensource.krypto-it.pl/sources/kit-static-scgi