Are you for real complaining about memory use in a alpha version of Opera.
When you downloaded Opera 12 guess you must have missed the big red warning.
“WARNING: This is a development snapshot: It contains the latest changes, but may also have severe known issues, including crashes, and data loss situations. In fact, it may not work at all.”
Well I tried opening the same 99 pages in Opera 11.61 , Opera Next build 1256 alpha & Firefox 9.
Opera 11.61 used 2.276 GB
Opera 12 used 2.342 GB
Firefox used 1.239 GB plus a plugin container 715 MB = 1.954 GB
More importantly to me both versions of Opera loaded all the pages far quicker Firefox took about 3 times as long.