Some secure google pages garbled in Firefox

This morning suddenly I couldn't access the login page for my Gmail in Firefox (13.01) The page came up with a white background and nothing but nonsensical characters filling the entire screen (random letters and symbols mingled in with a lot of little black diamond shapes, for example: H8o>�-�q-��|h{�=~gG ��). I Googled the problem and the search results came up with the same type of nonsensical characters filling the screen. I then tried my Google Calendar and Reader—same thing. After some experimentation, I discovered it was any Google-related pages from secure URLS (https://) that display this way. If I remove the "s" (making it http://) on URLs that allow it, the page displays properly.

This was not happening yesterday. Nor does this happen in IE9 or Opera.

Anybody have a clue...?

Also, I should mention that I cleared the browser cache, the Windows font cache, and followed the troubleshooting steps on the Mozilla site Standard diagnostic - Firefox - MozillaZine Knowledge Base and Standard diagnostic - Firefox - MozillaZine Knowledge Base