I got it to work in FF using the manual approach. Don't install automatically and don't upgrade when asked. When you see -""The o2c-PlugIn for Firefox should be automatically installed, when you visit the o2c-Startpage. If not, you should do the following.
Download the xpi-File: Download o2cplayer.xpi (as zip-archive) Choose
Extras -> Addons in the Firefox-Browser. A Windows opens, that Choose
Extras -> you can drop the xpi into. Click on "Install", when Choose
Extras -> the button is not greyed any more.
Now you can see o2c-objects on Sites, that have changed the html-Code in a way, that the Firefox-PlugIn is included."
Follow the part that I enlarged. Drag and drop the o2cplayer.xpi file onto the Add-ons window, you will get another window to install now. Should work after that.
Daniel Kaczor
DataCAD user since the big Cadkey fire sale.