One of our team members has spent time reading the page source and found the report rendering problem in Firefox.  I hope it's useful for your investigation.  Please see below.

The ReportViewer control creates a huge number of nested tables and div tags. Firefox 1.5 (though not FF 1.0.7) chokes on an empty table cell with a width of 100% that’s in one of these tables. This cell’s measurement attributes seem to take precedence over other sized elements and cause the columns in the report to crunch down to tiny unreadable widths. Removing this table cell from the DOM fixes the rendering error completely.

The specific table cell is located in the first nested table inside the div classed “ReportvieweroReport”. That div contains a table with a table-body element, containing a single row, containing two table cells. The first of these table cells has the id “mp01_bodyRegion__ctl1__ctl0_ReportVieweroReportCell”. The *second* of these two cells is empty, but is set to width=”100%” and height=”0”. This cell breaks the layout in Firefox 1.5.

(var ClientReportmp01_bodyRegion__ctl1__ctl0_ReportViewer__ctl2 = new RSReport(ClientControllermp01_bodyRegion__ctl1__ctl0_ReportViewer, "mp01_bodyRegion__ctl1__ctl0_ReportViewer", "mp01_bodyRegion__ctl1__ctl0_ReportVieweroReportDiv", "mp01_bodyRegion__ctl1__ctl0_ReportVieweroReportCell", "100", "", 1, 7, false, false, "", 0)