Motivated by this question: https://meta.stackoverflow.com/questions/368024/have-favorite-tags-always-been-alphabetized/
This'll restore the previous behavior: tags will remain in the order in which you add them.
But it'll even do one better: per rene's suggestion from the comments, it'll also allow you to reorder the tags at will by dragging them around (you have to start editing them first though, so as to avoid inadvertently reordering them while trying to click).
CC-BY-SA 3.0 of course
Get Tampermonkey or whatever you use to run userscripts, then install here: https://github.com/Shog9/ReoderableFavorateTags/raw/master/reorderablefavoritetags.user.js
Tested on Chrome 68 and Firefox 61
Works in Tampermonkey; probably works in Greasemonkey but untested.
Feel free to make useful changes & submit a pull request on GitHub.