SneakyWho_am_i wrote:

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http://members.shaw.ca/Limulus/files/w3sbw2-0801.png
Above: Firefox is now more popular than any version of IE, and will soon be more popular than all versions of IE. Firefox is a massive threat to Windows (a la Netscape 1995) as it allows clever programmers to develop applications directly for the browser, bypassing the Operating System. more users of standards compliant browsers means more webmasters coding for standards compliant browsers - which will make it more difficult for Microsoft to reverse the decline in popularity of their browser.
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Please note that my graph only shows stats *for that site* specifically; Firefox is almost certainly *not* the browser with largest market share worldwide.

Note however that there is disagreement about which is and what Firefox's market share is too:

http://marketshare.hitslink.com/report.aspx?qprid=2
Net Applications: February 2008
IE7: 44.03%
IE6: 30.63%
FF2: 16.30%

http://www.onestat.com/html/aboutus_pressbox57-firefox-mozilla-ie-browser-market-share.html
OneStat: February 18, 2008
IE6: 53.95%
IE7: 29.06%
FF ALL: 13.76%

http://www.w3counter.com/globalstats.php
W3Counter: February 29, 2008
IE6: 39.00%
FF2: 22.89%
IE7: 22.67%

The reason I graph the data from w3schools.com is that they have numbers going back six years and the major trends they show (FF rising steadily at the expense of IE, no major third browser uptake (IE+FF hovering at a stable, high level), IE6->IE7 adoption plateauing unlike IE5->6 adoption) *do* seem to be reflected across the web.

For a long-term graph of global trends, Janco has a decade of IE data:

http://limulus.wordpress.com/2007/11/17/jancos-browser-wars-graph-1997-2007/

I took their graph and added approximate release dates for IE and FF:
http://members.shaw.ca/Limulus/files/Browser_Wars_Janco.png

Note that IE's market share has been falling since *at least* mid-2004 and probably started at the beginning of 2003.

(Aside: these dates are interesting since they line up closely with the early development of Firefox; the first dev version, named "Phoenix" with version number 0.1 was released in September 2002 and the first "Firefox" dev version, 0.8, was released in February 2004.)

SneakyWho_am_i also wrote:

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NOTE: Google Trends is not a great source for meaningful statistics.
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You can learn a few things though; consider:

http://www.google.com/trends?q=Fedora%2C+Debian%2C+Ubuntu%2C+SUSE&ctab=0&geo=all&date=all&sort=0
Among major Linux distros, Ubuntu is the leader of the pack...

http://www.google.com/trends?q=XP%2C+Vista%2C+Ubuntu%2C+Mac&ctab=0&geo=all&date=all&sort=0
but still has a way to go before catching up to Windows and Mac OS in terms of being a 'household name'.

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I am always trying to convert people - but I don't push Firefox as hard as I can. I will defend Firefox, but I will support anyone who decides to try out Opera. The main thing is that they're not using Internet Explorer.
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But if they are going to use IE, make sure to push hard that they're not using IE6! :)

If you need to, direct them here:
http://forums.mozillazine.org/viewtopic.php?t=170212
(a bit old, but its still pretty much the same IE6 and just plain SCARY!)

Right now you can get Windows (XP/Vista) users to try:

IE7
IE8 beta

FF2
FF3 beta

Safari 3 beta

Opera 9
Opera 9.5 beta

Lots of choices; I'm sure one of those would do and chances are they'll pick something that's not IE.  Maybe do a 'browser-a-day challenge' with all of them ;)