Not a total fix, but I always just put a new rule under the one I'm replacing, and use the checks to enable (override) or disabled the rule.
Re:Firefox’s New WebSocket Inspector
by digitarald on Wednesday October 16, 2019 @02:31AM
(#21267646)
Proper undo-redo is on the roadmap. It works in some cases but needs to work throughout the whole Inspector!
It's amazing how such a small change makes it so much more difficult to use the Firefox dev tools compared to Chrome. Sometimes I just want to toggle between CSS values and see what is changing, or undo a few changes I made.