To use the HTML 5 <video> tag in Firefox 3.5, you need video files encoded in the Ogg Theora format. Apple doesn’t support this format at all, so you can’t just export Ogg files from QuickTime like you can with H.264/MPEG-4.
… and …
ffmpeg2theora is the one tool I found that simply just works for transcoding to Ogg Theora.
Firefox 3.5 FAQ – Nicely-written, and brought my attention to Fashion Your Firefox, a website which recommends add-ons for your installation.
Q: How can I customize Firefox 3.5?
A: Firefox is the most customizable browser on the planet. With a growing library of more than 6,000 free Firefox add-ons, little extras you download and install for a browser that works your way. Manage online auctions, upload digital photos, see the weather forecast in a glance and listen to music all within Firefox. If you’re not sure where to begin, try Fashion Your Firefox—a handy Web application that recommends add-ons based on your interests. Get it at: http://www.fashionyourfirefox.com