The ssrs reports is not showing any graphs in the thunderbird and gmail. But it works in the outlook without any problem. The graphs sent comes as an attachment and there is a rectangular box in the place of the graphs.
SSRS report sends email in different format and I believe you are talking about sending it as mhtml. Thunderbird doesn't support the viewing of mhtml format in its client, rather sends it as an attachment that you can view with add-ons. Gmail should open it though.
Hope this helps :)