Recent updates have nerfed most of the effectiveness of this add-on, which is sad, because it used to be an outstanding password manager. Now, it frequently spams you with remember-this-password requests when they are not needed, but frustratingly refuses to remember passwords, particularly when you've just asked it to create one! (Older versions would save the password without assigning it to a site, so you could add the rest of the site info manually if necessary; this version just ignores the newly-created password completely.)

Auto-fill sometimes works and sometimes doesn't, and there's no convenient shortcut to copy a password so that you can paste it somewhere else (say, an app that LastPass doesn't directly interact with). This isn't just an issue with New Firefox (although some of the bigger issues exactly match the issues that the add-on has in Chrome, so that's part of it), but more an issue with what seems to be the direction of the company - spending a lot of time on marketing and not very much time on functionality.

The premium option was actually worth paying for at one time; I wouldn't (and don't) pay for the current version.

Unfortunately, as of January 2020 it's gotten even worse, as it seems to have significant problems with authentication - if you can't log in, then you can't use the add-on. Would love to see them roll back the most recent update or at least take a closer look at their error logs.