Firefox Lite, Mozilla Lite, Thunderbird Lite -- where to find them?

Hi to all

I'm finding the current versions of these browsers to be very taxing on my old hardware, an old 
celeron 400 mhz computer.

I don't have many additional plug ins etc loaded either.

Are there any lite versions of these programs around? Particularly if it's just basic functionality 
required?


TIA
,
Speedy

PS Netscape 4.7 etc used to work just fine for me. I wouldn't dream of using it now though, because 
of security risks, outdated etc