Well - it works, but beware that when you're on the Free plan, you have a limit of 500 annotations, and it's not mentioned on their webpage or during the sign-up process, which is shady.

Keep in mind that you can still make annotations after you hit your limit, but making a new one will force you to dismiss a popup box that tells you that you're over quota, and will nag you every time you visit their webpage.

The premium subscription is priced ridiculously, at 40$ a year. It certainly DOES NOT cost them 40$ a year to host small text files and a few megabytes of cached content. Developers should be paid for their work, but what they're charging is far too much for such a basic service. In contrast, you could get 100GB of Google Drive storage for 25$/year, and Apollo Ultra costs 10$/year, and that developer actually communicates, updates the app and is transparent about their billing practices.

Not only that, but updates are extremely rare (just take a look at their blog page, the last post is from May 2018), and their "Groups" social feature looks like it hasn't been updated since 2008. It's also filled with spam.

All I want is an extension that allows me to annotate without connecting to any cloud-based service (Seeing how they barely update anything, security is a concern).

Alternatives to Diigo are relatively hard to find. I would have thought that extensions that let you highlight, annotate and tag would have been more commonplace, but the only actual alternative I found was hypothes.is. That service is updated much more frequently, and costs nothing for non-LMS uses. The main drawback to that, however, is the rather clunky UI that doesn't play well with NoScript and the lack of an official Firefox extension, as well as the constant cloud connectivity.