...description... Firefox version ... FIREFOX ... Firefox version

Opera

Location: http://www.mvps.org/dmcritchie/firefox/#skel.htm
Home page: http://www.mvps.org/dmcritchie/firefox/firefox.htm
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Tasks to which Opera is much better suited than Firefox

You can use the Launchy extension to bring up Opera for things that Opera does much better than Firefox, even when Firefox has the extensions that I do use. 
  1. Find (Ctrl+F), keeps the first FIND centered in the middle of the page, subsequents Finds do not cause scrolling until needed, so that you can always see the string in context. [Bug: 171237 and [Ext] userChrome.js], better described in Bug: 412334 In fact with Opera Find ALL is done automatically as well and much faster.
  2. Watch out for signs of Firefox keyword (bookmark tag) and Firefox bookmarks in general becoming improved into uselessness as evidenced in early Firefox Betas.

Chronology

I keep a copy of Opera around as a backup browser, and have done so for as long as I've used Firefox, which I guess was in 2004.