This can happen to you, one day, your Firefox browser tab bar doesn’t show “plus’ ‘+’ button next to opened tab (unless you or Mozilla intentionally removes it) which we use to open the New Tab. Though you can still able to open New Tab with shortcut Ctrl+T or with a double click on space next to tab, that is tedious but there is still an easier way to add it back to Firefox tab bar.
Restoring missing New Tab Plus button in Firefox
1. Firefox Menu button>Options>Toolbar layout>Drag and drop “New Tab” button onto tab bar space where it used to appear in the past, not found New Tab button? go to step 2.
2. In Customize Toolbar window, click on “Restore Default Set”, Done.