You can install the Firefox add-on from here.
With this extension, tab titles will be prefixed with the domain name of the site open in it.
The domain name will be enclosed in brackets and will appear before the actual title (i.e., the tab title of www.seas.upenn.edu will be [seas.upenn.edu] Penn Engineering | Inventing the Future).
The second-level domain will always be shown. However, whether or not the third-level (and nth-level) domain will be shown depends on the total length of the full domain name. The www part will never be shown as a part of the domain name in the tab title (i.e., www.google.com will appear as google.com).
You might find this extension helpful if you've disabled favicons in your browser for whatever reasons (e.g., privacy concerns due to supercookies).
You can specify what text prefixes the tab title for certain domains through the preferences page of the add-on.
For example, to have stackoverflow.com prefixed with [SO] instead of the default [stackoverflow.com], you can enter the following in the preferences page:
stackoverflow.com=SO
You can specify as many exception rules as you like, each separated by a new line:
stackoverflow.com=SO
github.com=GH
courses.upenn.edu=Path@Penn