This is correlated with, but not for sure caused by, upgrade to Firefox v1.5.0.10 (via GUI Update Manager):
I suddenly had a problem with visiting secure sites; e.g., login page for many web mail accounts. Got error page from Firefox saying there was some kind of server/config error, and something to do with "Personal Security Manager." Only other thing recent possibly related was addition of a Windows box to home network, but seems crazy to think that is at fault. Nonetheless, probably have done more than one upgrade to Linux box since last tried accessing any https sites, so unsure.
A bit of Googling suggested that missing package "mozilla-psm" was at root. Couldn't find this package directly through GUI package manager or via apt-get. Came across it as an evident download page, on Ubuntu site I think, but install info lacking. Nothing glaringly obvious on a few Linux forums that I looked at.
Decided to download and install Firefox v2.0.0.2 (the lazy way out). Downloaded tarball, killed current instance of Firefox, then did:
$ sudo cp ~/Desktop/firefox-2.0.0.2.tar.gz /usr/lib/.
$ cd /usr/lib
$ sudo mv firefox firefox-v1.5
$ sudo tar zxvf firefox-2.0.0.2.tar.gz
$ sudo rm firefox-2.0.0.2.tar.gz
Note the "mv" command refers to the directory firefox, and note that every line beginning with a $ should be entered as a single line.
Existing symlink (from v1.5) has /usr/bin/firefox pointing to /usrlib/firefox/firefox, so launch mechanisms preserved.
Everything worked after that: Add-ons updated properly on initial launch, can now access secure sites, etc.
Probably also worth noting that Upgrade Manager may not handle new version of Firefox properly, since v1.5.0.10 is what's current as an Ubuntu package. AFAIK: Firefox v2 hasn't been packaged for Ubuntu nice and easy upgrade/install yet; they're still on v1.5.x. FWIW: Update Manager thinks my system is up to date when force check; i.e., at least doesn't complain about new version. I'll have to try to remember to note what happens the next time an upgrade for either version is available.
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Update
2007-03-14 08:11 EDT
There are some more comments about this on a related post on my other blog.