We have some news regarding Firefox 7 compatible XPI!
In order to let you guys use the Yummy Player on FF 7.0, please download the following file with Firefox and install it (double-click on the file once downloaded).
<URL has been removed>
If the .xpi is not recognized and you are asked to choose a program to execute it, choose Mozilla Firefox.
Once installed, close and reopen Firefox, login on the site and ...play

For info, is not live right now but it should be very very soon.
EDIT Oct 3rd:The XPI has now been embedded in the Player, so a fresh download and install of the Player will fix it.