All software has bugs, even the best software has bugs. Though they have been quickly patched and dealt with, both Opera and Mozilla/Firefox have had some serious problems this month - everyone has patched haven't they?
Since I've got Opera's adfiltering working at full tilt, I'm using Opera again, after a few months with Firefox. One thing that does annoy me though is that sometimes, with some sites, Google in particular, the Opera and XFree processes go mad and eat up 100% of available RAM. The CPU load during this little sulk is low, but as I've run out of RAM, the system thrashes like mad and grinds to a halt.
The nice people at Opera are currently tryig to figure things out for me. As it's intermittent, it's actually hard to do. Their current suggestions is to turn of plug-ins and Java. Given that I almost never use Java, this is no great loss. What I find puzzelling is that Google's site is so simple, that I can't see much in it to cause such a serious memory problem...