I just downloaded and started testing MemHT, but so far I am impressed. It has a nice and clean look to it and it runs very well. I found it very easy to setup and use to this point. It was also very easy to setup a new template for.
I will be continue testing it out and if my point of view changes I will surely let you know.
Firebug is my #1 web design tool for transitioning ideas and images into real site.
My favorite aspect of the tool is that it allows you to right click and inspect a section of a page you are interested in seeing the code for and it organizes the code for your viewing. Not only this, it allows you to edit the code live and see the changes immediately.
When it organizes code for viewing, it does not show you every line at once, it groups divs, paragraphs, etc for the best viewing. Alongside the page code it also lets you view the accompanying css (which is wonderful). Along with these features it highlights the section of the page you are viewing code for to avoid any mistakes.
Along with this, it has the console that allows you to monitor Ajax requests, etc, simply to find the kinks, etc that could be causing problems for your backend.
My only drawback at this point is that it doesn't play so very nice with Firefox. It does cause increases in memory consumption by Firefox but besides that it is absolutely wonderful.