* '3-bar' menu button (or Tools menu) > Add-ons I have attached some screen shots illustrating the about:config page and the change you should see on the Add-ons page. (3) Double-click the preference to switch it from true to false - note that this is for Firefox 43 only (2) In the search box above the list, type or paste xpin and pause while the list is filtered

Click the button promising to be careful. (1) In a new tab, type or paste about:config in the address bar and press Enter/Return. 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. The disabled ones are at the bottom of the list on the right side. '3-bar' menu button (or Tools menu) > Add-ons.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).

Last pass extension firefox