I noticed this also. But, after first veryifying that my Firefox 8 was up to date with the latest updates, I then went to http://watch.slingbox.com and when I attempted to login to my administrative account, the slingbox webpage gave me a message that an update to the sling player addon was required. And, after following the installation instructions that followed, I was able to view the slingbox stream again in Firefox as normal.
BTW, the same kind of update was required to view my SB stream again in Internet Explorer.
As I understand, you want to know if it is possible to get your Slingbox PRO-HD up and running with Firefox 8.
Unfortunately, not for the moment. Firefox for Mac computers is supported only on the previous 6 and 7 versions and at this moment we do not know if it will be supported any time soon.
In all case, I recommend you to check this link in order to get further information about the supported Operating Systems and web browser versions.