Curious, I have an intermittent problem with the clipboard under XQuartz, the Mac version of the X.org X11 server. I thought it was something in the server acting up related to its sync feature with the host clipboard (which adds another layer of complexity). The symptoms are the same for me: the clipboard remains stuck on the last thing copied successfully and will only paste that.
Until I caught a Qt error message: "QXcbClipboard::setMimeData: Cannot set X11 selection owner". On a hunch I checked clipboard functionality in a lowly xterm and sure enough it worked, and newly started Qt applications were fine too. Evidently Thunderbird is not a Qt application so it may be a coincidence that restarting the application(s) worked for me and Stephen. Also, building and using Qt's XCB (X11) backend is somewhat of a hack on Mac. But the actual culprit may be something in libxcb. R.
