dpkg: firefox: dependency problems, but removing anyway as you requested:
 fish depends on lynx | www-browser; however:
  Package lynx is not installed.
  Package www-browser is not installed.
  Package google-chrome-stable which provides www-browser is not configured yet.
  Package firefox which provides www-browser is to be removed.

(Reading database ... 274687 files and directories currently installed.)
Removing firefox (49.0+build4-0ubuntu0.16.04.1) ...
update-alternatives: error: /var/lib/dpkg/alternatives/x-www-browser corrupt: unexpected end of file while trying to read slave link
dpkg: error processing package firefox (--purge):
 subprocess installed pre-removal script returned error exit status 2
update-alternatives: error: /var/lib/dpkg/alternatives/x-www-browser corrupt: unexpected end of file while trying to read slave link
dpkg: error while cleaning up:
 subprocess installed post-installation script returned error exit status 2
Errors were encountered while processing:
 firefox
E: Sub-process /usr/bin/dpkg returned an error code (1)