This is an easy way to add back in the missing functionality from the left navigation from CRM 2011.
Display and navigate between tabs on the current form (handles tabs being shown and hidden).
Display and navigate between related entities and back to the main form (handles navigation items being shown and hidden).
Options to only display tabs or only display related entities, or display both.
Options to display only certain tabs and only certain navigation items.
The HTML web resource in the solution can be embedded in the footer (recommended) or header to display the additional navigation links.
CRM 2013 & 2015 On Premise or Online
*No warranty expressed or implied - use at your own risk!
Feel free to donate if this saved you some time or helped out :)