Cookies are both a blessing and curse on the internet. They are essential for anything you sign in to. So shopping, webmail, social networking, etc. All useful things, all need cookies*. Unfortunately, they can also be used for tracking by other sources, that may not have your best interests at heart. Advertising companies, governmental agencies, hacking organizations, etc.
The rest of this post will speak to how to manage cookies in Firefox (version 48 in particular).
In general, if you don’t know that you need a cookie, you’re probably better off not having it. So the default stance should be to deny all cookies. Then, for those sites you choose, add them to an allow list.
To do this, go to Options (menu button, options) and then Privacy section (looks like eye mask). Or put about:preferences#privacy into the address bar. Once there, set history to “Use custom settings for history”. Make sure “Accept cookies from sites” is unchecked. You can adjust any of the other settings to match your needs, but we’re focusing on cookies.
With that change, as you go about browsing the internet you won’t be accumulating cookies. When you come to a site that needs them (perhaps google.com for checking your gmail) you’ll need to make an exception. To do that, you copy the site name (most likely from the address bar) and then go back into the Privacy Options.
Once there, click on “Exceptions” (should be on the right). Then paste the address that you want to allow into the ‘Address of website’ box. Click “Allow” (right edge of next line down), and then “Save Changes” (bottom right of window). At which point, that site can set whatever cookies it wants, but no others can.
Overtime this method creates a list of sites you need cookies for, while blocking all the others. Not the simplest of methods, but it works.
For historical reference: Firefox (before version 44) had a feature where you could set cookies to “Keep until” “Ask me every time”. That would cause a pop-up on new cookies. You could then set the check box to “use my choice for all cookies from this site”, and then either allow or block. The end result is the same as the method described above, except that it gave the choice as new cookies were encountered.
But past is past. Below are links to support pages discussing this change.
https://support.mozilla.org/en-US/questions/1101070
https://bugzilla.mozilla.org/show_bug.cgi?id=469260
* No, technically don’t need cookies. In theory, the developers could have used a different method to handle logins. And perhaps in the future they will, but for now it’s cookies.