If you check your LastPass extension on the Add-ons page, does it say it was disabled because it cannot be verified? I have attached an example of that kind of message (first screen shot). To check the Add-ons page, either:

• Ctrl+Shift+a
• "3-bar" menu button (or Tools menu) > Add-ons

In the left column, click Extensions. The disabled ones are at the bottom of the list on the right side.

LastPass likely will get its extension signed soon -- or the update might already be available, please check for that -- there is a link in the next reply -- but in the meantime you can make a global exception to the signing requirement; there's currently no way to only make an exception for specific extensions.

(1) In a new tab, type or paste about:config in the address bar and press Enter/Return. Click the button promising to be careful.

(2) In the search box above the list, type or paste xpin and pause while the list is filtered

(3) Double-click the xpinstall.signatures.required preference to switch it from true to false -- note that this is for Firefox 43 only

I have attached some screen shots illustrating the about:config page and the change you should see on the Add-ons page.

Firefox 43 requires all extensions to be signed. See https://wiki.mozilla.org/Add-ons/Extension_Signing

The Lastpass extension is signed, and they haven't yet figured out what caused it to fail verification for some people. Uninstalling Lastpass via the Add-ons Manager and reinstalling from AMO solves the problem. https://addons.mozilla.org/en-US/firefox/addon/lastpass-password-manager/

Double-click the xpinstall.signatures.required preference to switch it from true to false

This is not needed as Lastpass is signed. See https://blog.lastpass.com/2015/12/easily-enable-lastpass-in-firefox-43.html/