Before Getting Started
Before we get started on how to clear cookies in Firefox on a Mac, you should first make sure that you are running the latest version of Firefox. Although software updates can be inconvenient, it's always a good idea to stay updated with the latest version of Firefox to assure that you have the latest bug fixes and security patches.
To get started, make sure that Firefox is open, and is the current active Window within OS X. From there, select Firefox from the top OS X toolbar, and click Preferences from the drop down menu. Once the Preferences window pops up, click the Advanced tab.
You should now see a link labeled Update. Click it. Upon doing so, there will be several update options for Firefox, such as Automatically check for updates to: Firefox, Add-ons, and Search Engines. It is a good idea to check all of these boxes, so that Firefox will automatically give you updates, whenever a new version is released. Under the When updates to Firefox are found: section, select Automatically download and install the update. After you have done this, you can close the System Preferences window.