The following notice from Twitter greeted users on April 2:
We recently learned that the way Mozilla Firefox stores cached data from Twitter may have resulted in non-public information being inadvertently stored in the browser’s cache. For example, if you downloaded your data using Firefox, the browser may have retained a copy of the download for a period of time. We have made changes to prevent this from happening again. For more information please see this post. If you have any questions or concerns regarding this incident, you can contact Twitter’s Data Protection Officer by completing this online form.
The blog post explained that the cache handling means that “if you accessed Twitter from a shared or public computer via Mozilla Firefox and took actions like downloading your Twitter data archive or sending or receiving media via Direct Message, this information may have been stored in the browser’s cache even after you logged out of Twitter. The Mozilla Firefox browser’s cache retention period is set to 7 days and after that time the information should have automatically been removed from the cache. This issue did not impact people using other browsers like Safari or Chrome.”
Twitter has addressed the issue by implementing a change so that “Mozilla Firefox no longer stores Twitter data in the cache at all. If you use, or have used, a public or shared computer to access Twitter, we encourage you to clear the browser cache before logging out, and to be cautious about the personal information you download on a computer that other people use.”
That last bit is good advice, regardless of what site you are accessing or what browser you are using.