Sasha,
See
http://www.mozilla.org/products/mozilla1.x/ for the Mozilla Package (browser, email),
http://texturizer.net/firebird/index.html for Firebird browser, and
http://texturizer.net/thunderbird/ for email client.
I have not tried Opera lately, however I want a
free browser and do not want
ads. Therefore, Opera does not meet that criteria ($39.95 to not see built in ads). All Mozilla products that I mentioned are 100% free and no built in ads or spyware.
Firebird and Thunderbird can be customized to your heart's content. There are 163 extensions available to do this. Their popup blocker is excellent and you can add the adblock extension which blocks all of those annoying ads on pages. Next, there are different search extensions you can add
http://mycroft.mozdev.org/ with different categories available. These can be used as a regular search or added to the context menu (highlight a word, phrase, web page and open context menu for search and get definition, search, translation, url page info, etc.).
I could go on and on but why not visit the pages I mentioned and try it. Oracle, IBM, and others are going to use Mozilla in their applications instead of MS. Another plus is that Mozilla is open source and people all over the world can contribute ideas, test it, and develop extensions, etc. to work with. Hey Sasha, you might even be interested in developing skins for Mozilla. Some of theirs leave a lot to be desired.