Earlier this evening, I was reminded of just how primitive the HTML5 support was in Firefox 3.5/3.6. While we have seen three major version releases since 3.5, it was actually still the latest version of Firefox less than 6 months ago (and was that way for almost 2 years). Therefore, Firefox 3.5.x still holds a decent amount of market share (probably as much as, if not more than IE6 did a year or two ago). Looking at a handful of websites for which I have analytics data, versions of Firefox prior to 4 still accounted for anywhere between 2% and 15% of the total visits to those sites last month. With all of that information, it’s probably still important to make sure your sites work in versions of Firefox as far back as 3.5.
There are two somewhat major gotchas in the way Firefox 3.x handled HTML5. The first is easily fixed with a few lines of CSS. The second can only really be fixed if you rewrite some of your HTML.