Does userChrome.css really require the @namespace url... line?
RE: @namespace url("https://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul"); /* only needed once */
I edited some of the appearance of Thunderbird by creating a userChrome.css file. Since I couldn't find a clear explanation of why the above referenced line needed to be included, I left it out, to no apparent ill-effect.
Now I'm wanting to customize the Bookmarks menu in Firefox, and I run into the same advice to include it, so I'm curious enough to want to find out if it's truly necessary or just some programmer's cute obsession with Ghost Busters.
Wšě wotmołwy (3)
Yes, you need this line to avoid possible namespace issues and for safety reasons.
Does this rule still apply when using Firefox 57? If so, what is the correct form to use since (presumably) XUL no longer applies?
Wot ender21 změnjeny
Firefox's interface is a mix of XUL elements and HTML elements. When you use ONLY the XUL namespace, your userChrome.css file cannot style the HTML elements.
That's mostly fine, but occasionally it's an issue. Since I got stung by it once, I no longer use a namespace. If you are worried about your rules bleeding over into areas you hadn't intended to style, there are other workarounds. I have brief discussion of this issue here: https://www.userchrome.org/adding-style-recipes-userchrome-css.html#namespaces