Using Firefox on Windows, I toggled the user interface to 3.0, then closed the local LP vault view. Now the LastPass icon on the Firefox toolbar doesn't respond to clicks - it doesn't generate any user interface to allow me to view my sites or go back to the vault so that I can change the interface back to the 4.0 interface.
Already tried downloading and installing the latest FireFox add-on for LastPass, no change.
The only way I figured out how to fix this was to remove LastPass from the FireFox extensions (go to about:addons, then click Extensions). Then I searched for the LastPass extension from Firefix and installed it. This is an older version of the LastPass extension, but it works better than the new version.