I believe the way firefox allocates memory is a bit different. It takes a percentage of the system memory for cache and sometimes the paging
could take some time.

I constantly monitor traffic to/from my network/pc and I dont think
Firefox sends any/lot of personal information out.. (compared to IE)

On the other hand, I've seen Microsoft applications and IE reach out to
Microsoft sites periodically.
(they claim they dont send out any system specific information... but
thats not true)