I used to use that Auto HD, but it took a second to change the quality after opening the video and skipping ahead while this plugin was enabled caused some problems or just didn't work.
Atm I'm using this one -
Magic Actions for YouTube™. It has a lot of stuff in it, including adBlock, center and other things, but I only use it for the auto HD and country flags next to names in comments (I just like to know). The only thing I don't like about this is that it adds a bar with irrelevant buttons under the video and this is wasted space for me. But this plugin still causes problems for me. When I have it on, I can't disable auto-replay in Watch Later, and I absolutely hate it. But having to manually set the videos to HD is even worse so I still have to use it.
Even though Magic Actions already has a center function, I use this one instead -
Center'd - Center the new YT, because Magic Actions's center was a bit weird and sometimes didn't work properly. This one is more reliable I think.
I still wish to see the day when YT works properly for me.