I have been using Firefox for browsing the net unless the website doesn’t support Firefox (there are a few which I use often). I had reported in this blog about a strange problem I faced with Firefox while browsing (if you have not read it, click here). Of late this problem was coming out more frequently and hence I decided to try out Opera. Opera is almost similar to Firefox with some small changes in the looks ( I had used Opera previously).
When I checked the memory usages of both the browsers, I found that Opera took 17 MB of memory more than Firefox for same windows and websites (see picture above) and this could be because of various reasons. Further tabbing through the pages are more faster in Firefox than Opera.
Next I wanted to check the loading time for any site. For this I compared the time taken by both these browsers for different websites. Here I found that Firefox was bit superior to Opera as it took less time. Another thing I noticed was, Opera takes more time to load sites with lot of images and again this is relative.
After installing Opera, the Firefox problem seems to have solved and it has not occurred in the past 2 days. Sometimes competition does solve issues. 
NB: The comparison given here may vary in different systems and based on other conditions. I used Windows XP with 512 MB RAM for this comparison.
A complete evaluation of opera is coming soon there is much more than what you have said and that’s y I love opera
This is a pretty subjective test.
More comprehensive and objective tests with less relative findings can be found by searching google – I would suggest people reference them if this is a topic that interests them.
I use Firefox for day to day browsing, as Google’s web applications don’t support Opera. But when I just want things to load quickly and be resident for a long period (like reading and referencing documentation), Opera is my browser of choice. It doesn’t suffer the memory leaks Firefox does.