I found that he vault works with Chrome but not with Firefox.
same here
Same issue here. It works if I allow LastPass in Private Mode and then open my vault once I'm in Private Mode. It also works if I log out of LastPass and then log back in. For logging out/in it doesn't last though. A drastic measure you can take is to turn off automatic updates on the LastPass add-on and then find LastPass in the Firefox add-ons page. From there install the previous version. As long as you've disable automatic updates then it should work with that previous version. You just have to be sure to check frequently to see if an update is posted and then remember to re-enable automatic updates.