As someone with lots of tabs (multiple hundreds) and windows (usually about two dozens) open in a session, I was very surprised and amazed how quickly the latest few Firefox versions started up. Before, there would be this constant fear of having to close the browser more often than every couple of months (due to updates or high ram usage, which also seems to have gotten better) because it would take several minutes of waiting idle until all windows and contents were slowly loaded and usable, whereas now everything is available in a minute or two at worst.
There used to be one such Firefox addon that even made it into the Debian (Wheezy/Jessie) repositories. Unfortunately it was written for the old plugin system and never updated and is now designated as blocking the new Firefox multiprocessing, so I already started looking into Firefox addon programming just to replace it. Somehow I never thought about looking for newer alternatives in the official addon store, where several modern ones seem to exist already, so I'd like to thank you a lot for your suggestion.