Inspired by this feature request, I decided to make this UserScript that allows you to favorite pools and sets.
It's very simple and doesn't require a whole lot of explanation, but first here's how to download:
1. Download tampermonkey for whatever browser you're using.
2. Click here
3. Click install
Here's the source code: https://gist.github.com/DontTalkToMeThx/00b47382b7ddc54e01c58a23ef4b7d15
How to favorite/unfavorite?
Go to the set/pool you want to favorite/unfavorite, and click the button that says "Favorite" or "Unfavorite"
Where are my favorites?
For pools: https://e621.net/pools
For sets: https://e621.net/post_sets
Like I said, it's very simple.
I have a bunch of tools listed on my user page. If you have any ideas for a UserScript, send me a dmail. If it's simple enough, I'll probably do it.