Comment by kbelder

I added

  border-radius: 0px !important;

to my user chrome as per the article, but it didn't seem to accomplish anything. I've restarted with no effect. I didn't have a custom userChrome.css file before, but the instructions were clear enough. Do you have to enable it or something?

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Ok, found that you need a setting in about:config

  toolkit.legacyUserProfileCustomizations.stylesheets = true