June 2, 2014, 5:49 p.m.
The latest Firefox no longer has an add-on bar which also caused the ForceCORS button to disappear. Github user lissyx was quick to fork it and rewrite the extension to fit the new Mozilla SDK but as we tried to submit the update to the Mozilla add-on site it got rejected due to security considerations. I'll now try to find a way to bring it back to the official add-on site by making a few changes requested by Mozilla but until then, here's a workaround:
Download the Addon Bar addon
Since a lot of people miss the add-on bar, a new add-on called the addon bar (restored) brings back the add-on bar : ) If you add this add-on, the ForceCORS button becomes visible again without configuration. You can use the customize menu to move the button elsewhere but you need to keep this addon installed to keep it there.
Install ForceCORS manually
If you've lost the ForceCORS extension after it was removed from the Mozilla add-on site, you can re-install it manually by downloading the XPI-file here and then opening it in Firefox.
Thanks everyone for reporting and discussing the issues - please continue to do so here. I'll try to get it back to an official add-on as soon as I can.