If you copy data from a private browsing window in Firefox 32 or earlier to the clipboard and close the private browsing window afterwards, you will notice that the data is not there anymore.
Firefox empties the clipboard whenever you exit private browsing mode. You can test this easily by opening a new private window in Firefox using the shortcut Ctrl-Shift-p, copying text displayed on the introductory page, closing the window again, and trying to paste the copied text to a regular browser window or text document.
The result is that it won't work since there is no data to paste anymore.
Why is that done? The main intention appears to be to protect data from surviving accidentally when the mode is exited. Opponents have argued that Firefox should not interfere with operating system specific features, that users should get to choose whether they want the data to remain, and that it is a feature that needs to be activated by the user which in turn means that the user should be aware of it.
The problems that some users have with the behavior is that it makes things more complicated for them. Instead of copying something in private browsing mode, closing the window and pasting the copied data into another application, they need to paste first and close later which means more switching on their part.
The clipboard handling when existing Firefox's private browsing mode has changed in Firefox 33. Instead of deleting the clipboard on exit, data is now retained so that it can be copied as expected.
Firefox 33 is currently part of the Aurora channel. It will take roughly 12 weeks before the feature lands in the stable version of the web browser. It may also take a while before it lands in the Aurora channel itself. For now, it is enabled in the most recent Nightly version of the web browser.
How are other browsers handling this?
- Chromium and Chromium-based web browsers (including the new Opera) are not deleting the data when you exit the private browsing mode.
- Internet Explorer does not clear the clipboard either.
What if you liked the original behavior?
There is no configuration switch or option to retain the original functionality. You can either avoid copying any data to the clipboard, or, if you need to, copy dummy data in the end before you close the private browsing window.
If you are using Windows, you may run the command echo off | clip using a command prompt to clear the clipboard.