I installed firefox 11 on my older machine and it consumes about 30 to 40 megabytes of RAM. Version 23 which was installed by default took at least 150MB.
WHAT IS GOING ON?
Comparing them by functionality there does not seem to be a big difference.
By the way, Even version 17 needs about that much, so the problem does not lie at the particular 23rd version.
But why is there so much more memory needed for the newer versions?