Type about:preferences#privacy<enter> in the address bar. The button next to History, select Use Custom Settings.

Turn on Remember My Browsing And Download History At the bottom of the page, turn on Clear History When Firefox Closes. At the far right, press the Settings button. Turn on ONLY Cache and Form And Search History leaving the others off.

So I can't keep just two cookies then, I have to keep them all, right?

The method I use, which is built-in, is to set all cookies to expire naturally when I exit Firefox. If there are cookies I do NOT want to expire, I create an exception so those sites are allowed to set persistent cookies. There are less than a dozen. Then I do not set Firefox to clear cookies when I closes. Here's how that is set up:

Go to privacy settings:

• Windows: "3-bar" menu button (or Tools menu) > Options
• Mac: "3-bar" menu button (or Firefox menu) > Preferences
• Linux: "3-bar" menu button (or Edit menu) > Preferences
• Any system: type or paste about:preferences into the address bar and press Enter/Return to load it

In the left column, click Privacy & Security

On the right side, if you don't have "Firefox will: Use custom settings for history", select that option. Now you can customize

☑ Accept cookies from websites (selected)
Accept third-party cookies: Always
Keep until: they expire I close Firefox
☐ Clear history when Firefox closes (not selected -- or in Settings, don't select Cookies)

When creating "Allow" exceptions, you need to include the protocol used by the site (http or https). So for example:

• https://support.mozilla.org

You can also use the Permissions panel of the Page Info dialog to create exceptions. While you are on the site where you want an exception, you can call that up using any of these:

• right-click a blank area of the page and choose View Page Info > Permissions
• (menu bar) Tools menu > Page Info > Permissions
• click the padlock or "i" icon to the left of the site address, then the ">" icon, then More Information > Permissions

Scroll down to "Set Cookies" and uncheck the "Use default" box, and then select the permission you prefer (Allow in this case).

This dialog saves as you go, so you can close it after that.

Sometimes that doesn't work right because the site uses multiple servers (e.g., accounts.google.com and mail.google.com) and the cookies are set on the base domain (e.g., google.com). So in that case you still need to create the exception by hand.