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Firefox has "Ubuntu" placed just before "X11" while Chromium has it placed later on in the string (after "(KHTML, like Gecko)").

There have been instances of web sites not recognizing Firefox only when the distro is Ubuntu: in two cases (http://www.summitracing.com/ and http://translink.com.au/), it appears that the web master fixed the issue somehow. The third instance (http://www.property24.com/) wasn't yet fixed when I last checked.

In the first and third cases, the page concerned didn't open and users were asked to download something like "Mobile.aspx". In the second case, users got mark-up meant for mobile platforms.

Users of other Linux distros didn't have problems and those using a version of Firefox downloaded directly from Mozilla didn't have a problem either.

Users of Chromium on Ubuntu didn't have the problem either.

At the time of posting, http://www.property24.com/ still offers to download Mobile.aspx and doesn't open the page correctly if "Ubuntu" is present ahead of "X11".

If I tweak the user agent string to Mozilla/5.0 (X11; Linux i686; rv:11.0a2) Gecko/20120114, Ubuntu, Firefox/11.0a2, the page loads without issue.

The issue with http://www.property24.com/ is discussed here as pointed out by Ocky over here.

The "summitracing" issue was reported here.

Edit: bug 89774 relates to issues arising from introduction of "Ubuntu" into the UA string. A "patched" beta Firefox is available for testing. Details of the ppa are available at Chris Coulson's blog.

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Have you considered filing a bug report on this ? –  bodhi.zazen Jan 16 '12 at 5:39
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I would think that this is a problem of the sites you mentioned, not of Firefox. Sites like http://detectmobilebrowsers.com/ can perfectly well detect, that my Firefox is not a mobile browser. –  xubuntix Jan 16 '12 at 8:50

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To answer your question... There is no rationale.

There is no consistent set of rules for formatting of the User Agent string that browser vendors or package maintainers are following. So, no web site can rely on that string for accurate information when serving a page.

The problem you describe is caused by a poor development practice at the sites, not an error in the UA string.

ref: http://www.sitepoint.com/why-browser-sniffing-stinks/

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developer.mozilla.org/en/Gecko_user_agent_string_reference has details and examples. en.wikipedia.org/wiki/User_agent has this: "... most Web browsers use a User-Agent value as follows: Mozilla/[version] ([system and browser information]) [platform] ([platform details]) [extensions]." As noted in the edit to my question, bug 89774 (bugs.launchpad.net/ubuntu/+source/firefox/+bug/897794) has already addressed the issue and the current UA string is "Mozilla/5.0 (X11; Ubuntu; Linux i686; rv:10.0) Gecko/20100101 Firefox/10.0" for Fx10 beta (ppa:mozillateam/firefox-next). –  user25656 Jan 22 '12 at 13:24

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