What is Firefox extension and how and when do I use it. does it work with Brave browser or just FF?
One extension is this: Research Workflow with WebResearcherJS and Joplin but perhaps there are more than this one.
Brave is just Chrome dressed up, so you would probably need the Chrome extension.
For Joplin, when talking about the Firefox or Chrome extension it normally means the "Joplin Web Clipper". It is a browser extension that lets you copy all or part of a web page directly into Joplin as a note.
The Brave web site says about browser extensions...
The Chrome Web Store is a useful resource to search for new Chrome extensions. But note that you can use those extensions for any browser that relies on Chromium, the open-source language that underpins the Chrome browser.
For example, the Brave browser will work with any Chrome browser extension since they share the Chromium code. There are other places to find extensions, including downloading them directly from the publisher’s website, but if you’re running a Chromium-based browser, the Chrome Web Store should be the first place you look.
Joplin Web Clipper is in both the Firefox and Chrome web stores.
I believe that you can access the Chrome Web Store directly from a menu in Brave.