Starting with Mutt 1.5.7, you can have Mutt cache IMAP headers to improve IMAP performance considerably. Set the header_cache option in your ~/.muttrc:
set header_cache=/home/bob/.cache_dir
cacheconfigurationheadersimapmessagesmuamutt
You need to restart Firefox for the modifications of hosts file to take effect (if you edited the hosts file while Firefox had been already opened).
browsercachednsfirefoxhostsresolution