How to completely remove firefox so it won't update?

Remove the firefox and all the associated language packs with this command

sudo apt-get --purge autoremove firefox

It should remove firefox and all language packs for it. The output in my computer is shown below:

anwar@edubuntu-lenovo:~$ sudo apt-get autoremove firefox
Reading package lists... Done
Building dependency tree       
Reading state information... Done
The following packages will be REMOVED:
  firefox firefox-globalmenu firefox-gnome-support 

Your output may vary depending on the packages you installed. The important thing is you should see the uninstallation of these packages

firefox firefox-globalmenu firefox-gnome-support

Using autoremove uninstall packages which was required by firefox but is not required now after removing it. Without autoremove those packages will be left on system.

autoremove: removes any packages on your system that are no longer needed. As an example if I install package A, it might install packages B and C as dependencies. Simply un-installing package A doesn't automatically un-install packages B and C as well, they are left installed. apt-get autoremove searches your system for packages that have been installed as dependencies but are no longer used and removes them.

See this apt-get manual page for more info.

Hope this helps.

In a terminal you can list firefox related packages.

dpkg --list | grep firefox

Uninstall the unwanted packages.