Whenever Firefox updates itself it clears all my remembered passwords, so I'm forced to log in to those sites manually again.
Is there a way to prevent this? It's really annoying.
Go to file > options > privacy > Firefox will:
Make sure the drop down menu says Remember History.
If this is only happening with some sites, the cause may be this http://www.dailygyan.com/2008/08/force-firefox-to-remember-password-of.html
That website will show you how to disable Firefox respecting the "autocomplete=off" flag that some websites send. They respect it by default for security reasons.
Also, double check to make sure that "Remember Passwords" in the security menu is enabled (and doubly so when it comes time to upgrade your install).
If none of these work for you, you may want to uninstall Firefox, run CCleaner and select registry to clean your registry, then reinstall the latest version of Firefox from their website. Once you've done all that, make sure all the options are set to what you believe they should be and test it for functionality.