You will need to install a user script manager extension to install this script.
For a list of features, see the comprehensive introduction to the script in the Flickr Hacks forum.
Flickr Fixr debuted here as a userscript back in 2015. You can still install Flickr Fixr as a userscript from here, but it is not the recommended way any more. Unless you have very special reason to prefer or need the userscript, you should always install a version of the browser extension instead:
While I try not to break anything in the userscript, the browser extensions are my first priority. And all my development and testing today, is done with the browser extensions.
Also browser extensions allow you to turn individual features on and off independently. The userscript is all or nothing.
You might also want to try one of my other userscripts: