You can't blame Firefox for that
HTML 5 is a specification determined by the W3C, not Mozilla. The W3C is a standards consortium that includes companies like Microsoft, Apple and IBM as well as independent industry experts. ALL new browsers are expected to support HTML 5 if they are to be W3C compliant - and that includes IE, Chrome, Opera, Safari and Konqueror as well as Firefox.
As a web developer myself, I greatly approve of the changes in HTML 5. Most websites these days involve embedding application-level objects and having to rely on third-party proprietary plugins like Flash, Shockwave, Silverlight and Java has been nothing but a joke if not a nightmare. The new <audio> and <video> tags are an absolute boon to web developers because they allow a unilateral cross-platform presentation of multimedia content without having to waste our time and yours on proprietary plugins. Or having to code 4 different versions of a site to work with all the different incompatible solutions out there.
If you're that concerned about Web security in multimedia content, use NoScript, which blocks the new embedding tags unless you explicitly allow them.
Finally, while there will initially be security flaws with the new tags, when these flaws are discovered and corrected, the remedy instantly closes all doors across all sites. Compare that with the old plugin situation, where you could have many different flaws in different platforms and a fix for one did nothing to fix similar vulnerabilities in others. Adobe might fix a memory overrun bug in Flash but that would not fix a similar bug in Silverlight. Also, the standards in HTML 5 are open for anyone to see and fix, while plugins like Flash and Silverlight are closed and rely on their parent companies to fix them.
No, this is much cleaner. It's well past high time a multimedia standard was embraced by the W3C, and this hasn't come soon enough.