Comment 8 for bug 1624320

From the man page systemd-resolve can run in 3 mode of operations.

1) Ubuntu 17.10 default - The default is to list the 127.0.0.53 DNS stub (see above) as only DNS server. This file may be symlinked from /etc/resolv.conf in order to connect all local clients that bypass local DNS APIs to systemd-resolved. This mode of operation is recommended.

2) WHAT YOU WANT: systemd-resolved maintains the /run/systemd/resolve/resolv.conf file for compatibility with traditional Linux programs. This file may be symlinked from /etc/resolv.conf and is always kept up-to-date, containing information about all known DNS servers....

3) PRE-17.04: Alternatively, /etc/resolv.conf may be managed by other packages, in which case systemd-resolved will read it for DNS configuration data. In this mode of operation systemd-resolved is consumer rather than provider of this configuration file.

The fix:
root@localhost:~# ls -la /etc/resolv.conf
lrwxrwxrwx 1 root root 29 mar 7 20:20 /etc/resolv.conf -> ../run/resolvconf/resolv.conf
root@localhost:~# rm -f /etc/resolv.conf
root@localhost:~# ln -s /run/systemd/resolve/resolv.conf /etc/resolv.conf
root@localhost:~# ls -la /etc/resolv.conf
lrwxrwxrwx 1 root root 32 mar 8 07:30 /etc/resolv.conf -> /run/systemd/resolve/resolv.conf

Finally firefox started working properly along with all my command line tools...