New Tab has never been in the Tab contextual menu - I have multiple older versions of Firefox installed and it doesn't appear in Fx 38.0.5, Fx 41.0.1, Fx 47.0.1, or Fx 56.0.2. Wh would New Tab even be necessary in the Tab Bar context menu, with the New Tab + button sitting right there?

Unless you have an extension installed which added that contextual man item ...

It is very necessary, in fact I used it all the time.

If I want to open a tab next to a tab that I select, which is a function I used almost exclusively when opening new tabs, this is not possible with + button.

What extension add this option though? I looked and didn't find any for Quantum

Look in your Legacy Extensions/Plugins it may be there as what you are using.

When click a Tab can use Duplicate Tab to open a tab then go where you want to.

A couple of options here fore you to look/read https://www.google.ca/search?q=Firefox+New+tab&ie=utf-8&oe=utf-8&client=firefox-b&gfe_rd=cr&dcr=0&ei=nQ8XWpjcHtCCiAOtlIvIDw

Please let us know if this solved your issue or if need further assistance.

Shajirr said

What extension add this option though? I looked and didn't find any for Quantum

None that I have ever used.