The title is slightly misleading as this addon is not specific to Home Assistant.
I was watching the recording of frenk’s latest livestream and he showed an interesting way to display home assistant in the Vivaldi web browser. It has the ability to display web pages in the side bar. Neat! So I went looking for a Firefox addon to do the same thing. The one I found is called Webpage Sidebar.
I find this more convenient that having my Home Assistant page always loaded in a background tab.
I have not been able to find anything similar for Chrome.