From: "Frank Haber" 

A gecko-family browser will always be slower starting (much slower the
first time after boot).  It has to load components which Windows has
already loaded, ready for IE to use.  Remember - part of the operating
system?  Here's one reason why.

I found Firefox's pulldowns slowish on a slow machine.  They're written in
some sort of modular runtime/interpreter/intermediate-language
somethingorother.  Those have speeded up a lot in the preview-of-1.0 vs.
the 0.9 versions - they must have said byebye to a lot of debug code.

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