Re: 100 times more?
"I use Opera (12.16) or Firefox. I have about 20 tabs open. Opera takes 3.7 GB. Firefox takes 4.2 GB with the same tabs."
Suggest you look at the output of free and see how much total memory is actually being used. On this 8GB i7 with lots going on it shows 7.6GB (3.4GB used ignoring buffers and caches) and only 80MB swap used.
The value you are quoting for Firefox sounds like the virtual memory size. If I look at the amount used size for the just top 18 memory 'hogs' using the virtual size it runs to 40GB - showing how useless a measure it is.