I recently found the Reddit Music Player, which is a great tool to find and listen to new music from music-oriented subreddits (r/deephouse is my current favorite). My problem is, I usually pin the RMP tab to prevent it from getting lost in the many tabs I open on Firefox, but this makes it harder to find out the title of the currently playing song or access the controls.
Opening the tab in the sidebar is an effective solution, but the option is hidden in the bookmark settings and doesn’t work half the time, so I decided to use an add-on instead. The Open Link in Sidebar add-on does exactly what its name says by putting an option on the context menu when you right-click on a link.
The sidebar add-on seems to work best with websites that have responsive design. Fortunately, RMP is one of them, but for most sites, loading them on the sidebar only show their leftmost side, making them unusable. Coincidentally, the only other two websites on my bookmarks list that looks great on the sidebar are Webamp and Online Radio Box – both sites where you could listen to something.