One of the main things I miss is the LCARSTrek theme by KaiRo. Unlike any other LCARS browser theme, I found it to be usable on a day to day basis. Sadly it isn't available for Firefox.
> The HTML editor not so much, and didn't appear to get much attention.
I found it useful. Firefox's inbuilt HTML editor features are worse, they don't have floating table editing. Nowadays I use Thunderbird to write HTML whenever I don't want to do it by hand, almost the same thing.
I highly recommend SeaMonkey's HTML editor to anyone who would otherwise be tempted to write HTML in MS Word.
It doesn't have a ton of features, but the webpage will be readable both in rendered and in source form.
I used to use it heavily when creating MS Help .chm (Compiled HtMl) helpfiles.