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.