I ama user of Thebrain for about 15 years. For me this is the best tool to build a second brain. The extension - the brain-box allows you to capture information from within your browser to your brain. Thebrain is a fantastic personal knowledge Management system.

Updated review.
With the rate of change of technology - it took me 3 years for my cycle to come back to theBrain - and it has the stability it lacked back then - completely new code - and a super-fair ownership plan. And now it's pushing ahead with amazing, sometimes unique features - and one huge advantage: an original author's love and consistent vision and dedication to us - his fans and users. (That would be Harlan Hugh - a man who has more H's per letter than anybody.) And a still young software and concept master - which is good because we have at least another 20 years until he gets replaced by anybody.
So the BrainBox in FireFox is a simple plugin that DOES NOT REQUIRE A CURRENT SUBSCRIPTION - so your true ownership is not impaired. It sends a link to the BrainBox icon on theBrain toolbar, that when clicked shows a list that can be added to theBrain in various ways, that in turn when clicked opens in the internal browser. Voila - easy as meatballs to eat. So - it may not get through pay walls and subscibed sites - but that's a standard issue PIA. Unless you could select your choice of browser to view. (Workaround - script a link grabber. Thanks - I think I will.)
And thanks Harlan and the team for incredible effort - I think you've arrived at something very useful to entire segments of civilization and humans.

Flawless integration; love it.

This is a fantastic Firefox add-on for quickly sending web pages in Firefox to BrainBox without having to first open a brain and navigate to the thought before inserting the web page link. It also allows you to add websites from Firefox on other clients (e.g., at work) where you may not have direct access to your brains. (I did this today at work, adding two web pages to BrainBox. At home, the two web pages were in the BrainBox of TheBrain 10 and I effortlessly inserted them into the intended brain.