UMO uses browser detection to try to determine which version of some extensions you want to download. For example, the Enigmail Extension, you need a version for every OS, including Windows, Linux and OS X. Look at the Enigmail Page.
Well, I see a download for Windows, but I am using OS X. When you visit it, you might see it for Linux, or OS X -- its hard to tell, since the page is cached with a Squid frontend, you get the download link for whatever OS viewed the page before you.
The solution is to add a
Vary: User-Agent HTTTP header, to make the cache use a different version of the Cache for every User Agent -- and therefore show the correct download link. The bad news is that Squid doesn't support the Vary header. I geuss the only option is Apache 2.1.