Last week, Mozilla released Firefox 3.6, the lastest version of most web designers' favorite browser. We highly recommend the upgrade. Unlike the more minor releases, it appears that users are not automatically prompted for the upgrade, so you may need to download it manually. Go to Help > Check for Updates in Firefox, or (if for some reason you aren't yet using Firefox) visit www.getfirefox.com.
Firefox 3.6 also implements a few more HTML5 and CSS3 features, including gradients and background sizing. Until more browsers support them, you'll want to use these with care, but it's great to see these features getting out into the wild.
One HTML5 feature that you can start using immediately is the option to set a script to run asynchronously. (Caution: ironically, loading the page in the preceding link brings Firefox to its knees, at least on my MacBook Pro, delivering about 60 seconds of spinning beachball.) This can speed up load times for pages using third-party services, such as Google Analytics, that are invoked via remote scripts. By setting the async attribute, the rest of the page will load independently, so delays at the third-party server don't slow it down. In fact, Google has already provided an updated analytics snippet that runs asynchronously. Browsers that don't understand the attribute will just ignore it, so there's no compatibility issue.
Along with Firefox 3.6, you'll want to get Firebug 1.5. (Not using Firebug? You're working too hard. Stop right now and go get it.) The latest version has an assortment of new features, none earth-shaking but well worth the upgrade (which is required in any case for Firefox 3.6).