All my addons stopped working after upgrading to 3.6.12 on a Mac 10.6.5
After upgrading all my addons disappeared from the browser interface. I tried using Time Machine to restore the application to an earlier version which it did, to 3.6.10, including the profile folder in the Library and still none of them work. I brought up the addons window and they are all listed. I tried updating and was presented with a list of about five, which I then updated, but when installing I get the message "Firefox could not install the file at... because: Unexpected installation error. Review the Error Console log for more details - 203"
I found a solution. I uninstalled Firefox since I was using Chrome 90% of the time anyway.