Every site on Firefox "use an invalid security certificate"

I’m running OS11.1 with Gnome, and yesterday I wanted to try Firefox 3.6, so I went to the download page, found the one-click install for it (from Mozilla repo) and installed it.
It updated a bunch of other packages, and then I started the program. It wouldn’t start, giving out an error about not founding some GRE thing, or something.
So I shrudded and went to Yast and reinstalled the “official” Firefox 3.0.17 build.
Since then, each and every site that need secure connection won’t load, and the error page saying

Secure Connection Failed 

www.siteaddress.com uses an invalid security certificate.

The certificate is not trusted because the issuer certificate is unknown.

(Error code: sec_error_unknown_issuer)

    * This could be a problem with the server's configuration, or it could be someone trying to impersonate the server.

    * If you have connected to this server successfully in the past, the error may be temporary, and you can try again later.

          Or you can add an exception…

I tried adding the exception, but in some cases it won’t even work this way, and the error keeps showing. Also, I don’t think I can add exceptions to each and every secure server in the entire world…

Any suggestions about how to revert this problem?
Thanks a lot.