> Firefox no longer loads userChrome.css or userContent.css by default improving start-up performance. Users who wish to customize Firefox by using these files can set the toolkit.legacyUserProfileCustomizations.stylesheets preference to true to restore this ability.

How much of a performance improvement does this make?

Last time I tried, I just couldn’t get userContent.css to work at all. Maybe a user error, but I’d really like an easy way to set some default styles to override the inane Web 0.5 era defaults such as zero margins and unlimited-width paragraphs. Reading Good Old HTML (tm) pages sucks, and although there’s the reading mode, it tends to simply drop vital content it doesn’t happen to support…