Since upgrading to v 12 on 2 machines one XP the other Windows 7 when I select in twitter Firefox acts as if the left button has been either held down or clicked twice. Sometime in Yahoo mail I select a folder and it open the folder then as the moves the list of folders opens the folder which is now under the cursor. In Twitter simply crossing over a message can cause it to be selected. This happens on 2 machine with different OS and only since upgrade to 12 doesn't seem to affect other apps just firefox.