Just had sec_error_bad_signature on my old computer (it had FF 35.0).
Then I remembered that I had this solution, which worked again for me:

Quote Originally Posted by Robai View Post
Finally managed to log in from FireFox version 35.0.1 to these forums.

The solution that worked for me is described here:

1) That is, downloaded Certificate file from here
2) Then
Menu/Tools/Options/Advanced/Certificates/View Certificates/Servers/Import...
and selected the file

After that DDO forums page no longer showed "sec_error_bad_signature" page, but it showed the other page that was used to be in older FF versions, where I could select "I understand the risk" and then could add DDO forums page to the exceptions.
Now I have updated FF to version 37.0.1 and everything still works.
I'm not sure if I would have the problem if I would upgrade to FF 37.0.1 first, but anyway, if anyone still have the issue these steps should solve the problem.