Every time I go to add on the 1Password add-on, it will not do so. It doesn't automatically load the password as Firefox did before I added the 1Password add-on. Now, to enter a password, it's at least a 2-step process when before I tried to add this to Firefox, it was a 0-step process.

UX sucks. Blocking competing sign-in popups feature works really (not) great: it doesn't block LastPass when you enable it, and it blocks Firefox built-in manager when it's disabled.

The browser extension that has had the biggest positive impact on my browsing experience.