There's a lot of browsers out there that 'pretend' to be Firefox so that sites will work with it properly. Only a matter of time before someone does that for Chrome.
I remember reading that a lot of sites can't properly handle major version numbers greater than 9, so Opera 10 declares itself to be Opera 9.80...
Yep, it identifies itself as Opera/9.80 (Windows NT 6.1; U; en) Presto/2.5.24 Version/10.53 (or similar) because many sites detect Opera 1 instead of Opera 10. It's an example of why user agent detection is really a poor idea and you should just code to standards instead.
Before version 8.02, Opera browsers identified themselves with an Internet Explorer user agent string for that reason. This was actually pretty damaging for Opera support because developers would think everyone was using IE6 (which, any web developer can tell you, essentially has its own set of standards) and design pages with code that only worked in the Trident (MSIE) engine.
Pretty much every browser's user agent has Mozilla on it, and quite a few have "Gecko" (the rendering engine of Firefox & other Mozilla software).
User Agents are generally bloated because of compatibility.
Here's what a Chrome User agent looks like on Windows:
Mozilla/5.0 (Windows; U; Windows NT 5.1; en-US) AppleWebKit/533.4 (KHTML, like Gecko) Chrome/5.0.375.70 Safari/533.4
Mozilla/5.0
Mozilla could display frames, Mosaic could not. User agent sniffing started and browsers without Mozilla in the string got an error page or a page without frames.
Thus, MS (and later, everyone) started putting Mozilla at the beginning of the user agent string. MS followed it with (compatible...)
(Windows; U; Windows NT 5.1; en-US)
The most subject to change part of the string. OS Family, Security (U is strong), OS version (NT 5.0 is Win2k, NT 5.1 is WinXP, NT 6.0 is Vista, NT 6.1 is Win7), and language)
AppleWebKit/533.4 (KHTML, like Gecko)
The first part specifies the engine is based on WebKit and is a more "proper" way to sniff if you
must sniff the User Agent (you should just code to standards, but if you're sniffing to warn a complex app may not work that's acceptable).
KHTML was the source for the WebKit project, an earllier open source rendering engine. Sites would sniff for KHTML and would give WebKit errors or reduced content, so that stayed in.
Before THAT, sites sniffed for Gecko (Mozilla's rendering engine, basis for Firefox, Thunderbird, etc.), so the KTHML devs put "Like gecko")
Chrome/5.0.375.70
Browser and version number. Not hard to get.
Safari/533.4
Google stuck this on the end to try to get sites that sniffed for Safari to not say "HURR DURR INCOMPATIBLE BROWSER". Like Safari itself, it's Safari and the WebKit version.