OTOH, if I have Firefox open (with the usual mess of dozen of tabs
already there) opening even a very simple page like Lua refman's it
takes at least a couple of seconds. More if the other tabs are busy
doing something. Moreover, if Firefox has not been started yet, it takes
many seconds to display the page (i.e. forever when there are cached
tabs that have to be reloaded). This is quite annoying.
This is why I keep an old version of Opera (9.0.1) running on my PC all the time. It eats miniscule (by today's standards) amount of RAM with several tabs open. I use it as a document reader for HTML manuals.
I've modified the Lua HTML refman to have two frames: TOC on the left.
Works well for me.