This sounds like a mistake on my part (firewall/port taken), but this is a little trickier (or so I hope).
lsof -i :8888
COMMAND PID USER FD TYPE DEVICE SIZE/OFF NODE NAME
firefox 30120 vh 36u IPv4 8008134 0t0 TCP localhost,:8888 (LISTEN)
But I can not send my extension (I tested wget and curl).
wget --post-file=extension.xpi http://localhost:8888/
--2016-04-29 17:05:45-- http://localhost:8888/
Resolving localhost (localhost)... ::1
Connecting to localhost (localhost)|::1|:8888... failed: Cannot assign requested address.
Retrying.
--2016-04-29 17:05:46-- (try: 2) http://localhost:8888/
Connecting to localhost (localhost)|::1|:8888... failed: Cannot assign requested address.
Retrying.
Chain INPUT (policy ACCEPT)
target prot opt source destination
Chain FORWARD (policy ACCEPT)
target prot opt source destination
DOCKER all -- anywhere anywhere
ACCEPT all -- anywhere anywhere ctstate RELATED,ESTABLISHED
ACCEPT all -- anywhere anywhere
ACCEPT all -- anywhere anywhere
Chain OUTPUT (policy ACCEPT)
target prot opt source destination
Chain DOCKER (1 references)
target prot opt source destination