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You can preview a style in Firefox without installing it, just the ability to edit changed.
As to why that changed, IDK, but Jason did mention that other changes had broken it and he didn't choose to fix it. I believe Stylish first came out over 8 years ago and Jason has been consistently maintaining it since then. I also haven't noticed anyone stepping up and helping out with much on Github, even though he has mentioned help would be welcomed. With the changes occurring with Firefox, in addition to other projects Jason has taken on, simplifying things a bit may mean more time for other things that happen in the journey we call life.
Both of the extensions this thread covers add much functionalilty to Stylish as well.
Never said you can't complain, I do at times for sure. And a big Kudos to all the extension/add-ons developers out there. They do make my internet addiction more enjoyable.