Most of the challenges when working with CSS and PHP developement are "making it work" in all browsers. Previewing properly is the designers main concern and the difference in browsers makes things a little difficult. Some browsers have all kinds of bugs when it comes to previewing CSS, especially transparent PNG's. Yeah, I'm talking about you Internet Explorer 6. In order to get things to work properly designers are forced to write backwards compatible hacks, IE6 fixes and tons of extra code just to make sure the users experience is as consistent on your office PC as it is on the much beloved office/home Macintosh.
Upgrading your browser is usually one of the easiest things to do but some people love to hang onto what they know. I'm personally not a fan of Internet Explorer but if you must use it, use the newest version, IE7. Most of it's new perks are directly influenced by Firefox and it tends to render most CSS2 properly, making our jobs just that much easier. It's more secure than older versions and seems a bit easier to use all around.
On to my favorite - Firefox. Simply put, it's the best browser out there. It seamlessly renders CSS2 properly, has loads of add ons like Firebug, NoScript, Extra toolbars and plugins and so much more. On top of that you can easily customize the "look" of your browser by downloading Firefox Themes. No more shall you be stuck under Explorers blue thumb! Best of all it's open source so developers are constantly adding new tools and extensions to make life on the web a convenient and enjoyable experience.
Either way, upgrading your browser will help with both useability, security, consistency and performance. Older versions of browsers have been exploited, broken down and buggy and though you may still have some bugs here and there life's only getting better with every new version. So keep up the rest of us and happy browsing.