Good question - does the browser plug in sync to the internet or is any part of it internet accessible? I've not used it. I just know a lot of people are put off by the idea of their passwords being "in the cloud" or otherwise accessible through the internet. Looking at the add-on for Firefox, it looks like it communicates with the local keepassxc instance, which should be fine for many people.
Thanks. I was not aware of this option.