After updating to 126.96.36.199, Firefox crashes if the FoxyProxy extension is installed. This renders the browser virtually useless. You won’t even be able to install the update that fixes this issue once there is one, since Firefox‘ automatic update function does, of course, use SSL. Some of the links in this post also point to HTTPS URLs, so if you are reading this post using Firefox because you have this problem, read to the end but don’t click anywhere.
To fix this issue and get Firefox to work again, you’ll have to uninstall the FoxyProxy extension before accessing any HTTPS URL. If Firefox is configured to start with a blank or non-SSL page and you do not want to restore a session containing HTTPS pages, you can probably do so from within Firefox. Go to Tools -> Add-ons, select FoxyProxy in the list that opens, and uninstall. Or is disabling FoxyProxy sufficient? Please leave a comment if you tried; I didn’t.
If Firefox for some reason tries to load an HTTPS page automatically when it starts, you will have to edit your Firefox profile. On Windows, profile data are usually found under C:\Documents and Settings\<your username>\Application Data\Mozilla\Firefox\Profiles\<random name>\. Note that Documents and Settings and Application Data may be named differently in localised versions of Windows. Under MacOS X the profile is stored under /Users/<your username>/Library/Application Support/Firefox/Profiles/<random name>, and on a Unix-style system such as Linux or *BSD you will probably find the path to you profile somewhere under your home directory. If you see a recently edited file named bookmarks.html, and the file contains your bookmarks, you found the right place.
Inside the profile directory you should see a subdirectory extensions and inside, one or more subdirectory. Find the one containing FoxyProxy, it will probably be named email@example.com. Delete this subdirectory, or better move it elswhere just in case you made a mistake.
Now start Firefox again.
Update 2008-09-29: New versions of FoxyProxy are available. Installing version 2.8.6 or newer should fix the issue. The latest version is 2.8.8 now. If you haven’t updated your browser yet, it would be wise to update the extension first.