I did try to get a trial of Flash CS5 with Firefox 3.6.8 from adobe.com yesterday, but downloading page didn't open.
May be it's a temporary bug of abobe.com?
The place to report problems with the Adobe website is http://forums.adobe.com/community/general/adobedotcom_feedback (this here is the place to report anything concerning the Adobe forums).
When you do, please post the URL from where you tried to download your trial.
Generally speaking for issues with the adobe.com website you should post in Adobe Forums > Adobe general forums > Adobe.com feedback
However, specifically for issues with products downloads Adobe Forums > Adobe general forums > Downloads is a better choice.
Yesterday I tried to download trial Flash CS5 with IE8 and it's all OK. The difference, that in Firefox the popup window that has "Adobe installer" title and contains 2 links to download the archives did not opened.