This is not a firefox issue it is a MS issue with how they developed reporting services (the web based portal to access your reports). It is not compliant with some rules of the Web community, hence the issues you see. The issue pertains to the usage of iFrames. In firefox the iFrame's height has a small default value. In Internet Explorer, iFrames automatically resize based on a document's height. Not so clever.
See this: http://forums.asp.net/t/1135437.aspx
As well as this: http://blog-mstechnology.blogspot.com/2009/08/sql-server-reporting-services-ssrs.html
Mainly this:
To Fix the Issue in Firefox, hope any one the following 2 ways will help us.
Step1:
Go to the following Location, where SQL Server is installed
..\Program Files\Microsoft SQL Server\MSSQL.3\Reporting Services\ReportServer\Pages
Open ReportViewer.aspx File
add the style property marked in bold and blue color, Then try this
style="display:table; margin: 0px; overflow: hidden" ID="ReportViewerControl" runat="server" />
If not Helps the first step, then try the step 2.
Step2:
ADD the following code to the ReportingServices.css file (by default, it's found in "C:\Program Files\Microsoft SQL Server\MSSQL.3\Reporting Services\ReportManager\Styles\"):
.DocMapAndReportFrame{ min-height: 860px;}
If you want Width also, then use this code
.DocMapAndReportFrame
{
min-height: 860px;
min-width: 1000px;
}