I ran into this too a few months while back, oddly enough with IE. Some proxy server or something was caching old versions of CSS files. After messing around forever with HTTP headers, I found the easiest way to deal with this is the href="blah.css?i=12345" solution mentioned in the article. Works for JS files too.
I'm seeing this too, but it corrected itself somehow. When I started developing a site my CSS changes were not noticed without shift-reload, but now they are with a simple reload.
Odd, unless firefox detects update frequency somehow and decides if to do the reload based on that? Seems weird, but I never had a reason to investigate it much.
Rails makes this very easy with asset timestamps: http://api.rubyonrails.org/classes/ActionView/Helpers/AssetT... (well, "easy" sort of... until you have multiple web servers and the times get slightly out of sync)