hello, when you are using speedtest.net or a similar flash-based site to measure your connection speed it might be an issue with the protected mode that adobe has introduced for its flash plugin for firefox. some old versions of f-secure or trusteer rapport are known to cause such slowdowns together with flash, or it might be something similar on your system that is interfering (maybe security software that is blocking/sandboxing the container used for the protected mode, etc...).

as a workaround you could try disabling protected mode & see if this makes a difference.

in case you'd then like to troubleshoot the issue further, it would be best to contact adobe's support & file a bug report at bugbase.adobe.com (choose "Adobe Flash Player" as the product & "Browser Integration" as the product area), since they will have a better understanding of the internals of flash protected mode & therefore may know what else might be at the root of the problem you were seeing.

"Back in the day" when I used speed tests, the Java based tests were a lot closer on the various browsers, than the Flash based tests. And when I first tried Chrome (and it was using the same NPAPI Flash Plugin as Firefox uses) the tests results were always close to the same as with Firefox. IE was always faster with the Flash based tests with its ActiveX version of Flash.

Try a HTML5 based test. http://speedof.me/ IMO, that is the only far comparison of internet speed between the various browser; it rules out differences due to the plugin being used for the test.

Your System Details shows;

Installed Plug-ins

Adobe PDF Plug-In For Firefox and Netscape 11.0.07 Adobe PDF Plug-In For Firefox and Netscape 10.1.10

and

Version 0.9.19, Copyright 2006-2012 Veetle Inchttp://www.veetle.com/ Version 0.9.18, copyright 2006-2010 Veetle Inchttp://www.veetle.com/

Having more than one version of a program may cause issues.