Installing www-client/firefox-bin instead of www-client/firefox solved problem.

If you have other issues;

Do you have NVidia’s network access manager or Forceware Network Manager software installed? Disable / remove it. https://support.mozilla.org/en-US/kb/connectivity-crashing-firefox-43-update

FredMcD said

If you have other issues; Do you have NVidia’s network access manager or Forceware Network Manager software installed? Disable / remove it. https://support.mozilla.org/en-US/kb/connectivity-crashing-firefox-43-update

This is on Linux and not Windows ;) The article you linked to even says "This article only applies to users of Firefox on Windows" if a Linux user were to view it.

Linux is mentioned in OP and in right sidebar.

Also that article is for some other issues not related to right-click context menu.