This is more of a review of Lastpass generally, not just the Firefox plugin.
Palmharbor's review is immensely unfair. Lastpass works as it should almost all of the time and if a bug does arise, the developers are quick to respond.
As a password locker, Lastpass is as good as, if not a bit better than, 1Password. It saves site login information, generates secure passwords etc. This information, however, is stored on a remote server (encrypted) rather than in Dropbox or the like. Theoretically, this is more secure because if someone were to access your computer, the actual database would be inaccessible. Of course, there are plenty of counter-arguments to this.
Lastpass does have some drawbacks, however. The interface, wallet (for storing serial numbers for example), and desktop client are all quite poor, seemingly designed by engineers and not designers. The new interface for Lastpass plugins and mobile apps is a bit of an improvement, especially the plugin for Firefox. It annoyed me no end that I couldn't search for a site with the old Firefox plugin. If I needed to search, I had to open the Vault. Not so anymore. Still, the plugin is quite busy. The Safari plugin, however, is good. They've moved a lot of the menu items you see in the Firefox version to unobtrusive icons. They could certainly stand to learn something from the Safari plugin! The Chrome plugin, by the way, is similar to the Firefox plugin. I have had problems with the Chrome plugin for ages though. Sometimes it will not open, for example. I decided to blame Chrome for this though. It's a browser that just can't get its act together!
I mentioned that the wallet is poor. This is actually due to the horrible design of the desktop app. The desktop app, Pocket, looks like it was designed for Windows 95, with hideous and small little icons for navigation and little yellow folders to signal various categories. Unlike 1Password, you won't find individual icons for each app and you can't easily preview information about each app. For some, I'm sure this isn't a big deal, but for me I decided not to renew my Lastpass subscription because of it. I've been waiting instead for 1Password to update their Android app.
Which reminds me: Lastpass is $12 a year for a subscription. Pretty reasonable if you ask me. Also, Lastpass works well on Android. Extremely well in fact. The Android app includes a keyboard that will allow you to input login information etc directly into the app, no messing about going back and forth between apps. This is the big draw of Lastpass for me. 1Password's Android app is pathetic, but to be fair, most password lockers' Android apps are pathetic. Not so for Lastpass...