You need, need, need to Google places that have legacy versions of software, specifically classic versions of Firefox Add-ons. One I found is called the Wayback Machine. Of course, everything is unofficial & you download & use whatever you find at your own peril. But just now I almost installed VDH 6.3.1. Fortunately, Firefox prompts you before letting the installation of an add-on proceed so of course I told it to stop now. With a little more effort, I'm sure I could have found VDH 6.3.3. Personally, I say you ought to upgrade to modern hardware & software. But if you really are stuck in the past, there are ways you can help yourself.