I love that it hides but (by default) doesn't automatically unload/close a tab that you stash (it stays loaded in the background) because it allows you to switch back to the hidden tab without having to reload the page (so e.g. you don't lose your place in a video your watching). You can customize this behavior in the options menu or change it so that stashed tabs are automatically unloaded/closed. It might automatically unload running hidden background tabs under certain conditions but this behavior can be disabled in the options.
There's no picture of it, but the "Tab Stash" submenu in the (right-click) context menu has the items:
Show Stashed Tabs in a Tab
Show Stashed Tabs in Sidebar
Stash Tabs
Stash This Tab
Stash This Tab to a New Group
Copy Tabs to Stash
Copy This Tab to Stash
Options

Firefox's sidebar (that I previously hardly ever used) makes using this addon extremely convenient because I don't have to open/switch to an intermediate "tab management/list webpage" (i.e. a webpage that lists all tabs & tab groups, like OneTab's "Display OneTab" page).
[EDIT/UPDATE: Turns out you can using the sidebar] You can stash multiple selected tabs at once but I'm not sure if it's possible to stash them directly/immediately into a preexisting named tab group of your choice like you can do with OneTab (I think that stashed tabs either automatically go to the top-most group or it creates a new unnamed tab group - it seems to only create new unnamed tab groups for me but that may be because I've been fiddling with the options).
It would also be nice if there was a menu item like "Close this tab and remove it from all tab groups" so that I don't have to manually find and remove a tab (from the tab groups) once I'm 100% completely done with it.
Overall, I think this is superior to OneTab, which was my previous tab manager (I hear that Mozilla is working on finally giving Firefox its own tab manager so we'll see if tab manager addons become unnecessary).