Uninstalling Firefox installs Thunderbird?

gistfile1.sh

user@lubuntubox:~$ sudo apt-get remove firefox
Reading package lists... Done
Building dependency tree       
Reading state information... Done
The following extra packages will be installed:
  thunderbird                     <----------------------- WHAT????
Suggested packages:
  thunderbird-gnome-support ttf-lyx
The following packages will be REMOVED:
  firefox
The following NEW packages will be installed:
  thunderbird
0 upgraded, 1 newly installed, 1 to remove and 2 not upgraded.
Need to get 23.7 MB of archives.
After this operation, 16.2 MB disk space will be freed.
Do you want to continue? [Y/n] 
Get:1 http://ports.ubuntu.com/ubuntu-ports/ trusty-updates/main thunderbird armhf 1:31.2.0+build2-0ubuntu0.14.04.1 [23.7 MB]
19% [1 thunderbird 4,591 kB/23.7 MB 19%]                      212 kB/s 1min 30s
^C