This seems to be a regression as I'm hitting it a lot at the moment, but never saw it before. STR: 1. From a webpage press and hold the mouse on the scrollbar thumb, as you drag around the scrollbar will move up and down normally. 2. With the mouse still pressed move the cursor outside of the firefox window. 3. Release the mouse button. 4. Move the cursor back into the window. Now the scrollbar will still be following the cursor until you click somewhere.
This symptom is reported in bug 407876 comment #0 (along with several others). It's fixed by the patch for that bug.