Yesterday evening I tried to install a simple PDF printer (pdfforge PDFCreator) on my more or less new notebook. During the installation the installer seemed hanging somewhere and could not be cancelled. I also saw that the installer installed additional software (Anti Ad-Ware) which I didn't want at all. So I killed the installer process in the task manager.
Shortly after, when I used Firefox to search for alternative pdf printers, I saw that every new tab is now a Yahoo! search tab. Furthermore my search engine settings were emptied and only Yahoo! was in the list. A lot of other settings (like default home page) were changed. Anger!
Additionally to all that frustration it was not possible anymore to save any passwords in Firefox. In the Settings->Security window, the "Remember passwords for sites" box was even greyed out. Also the self-signed SSL certificates, which show up with a warning "Untrusted Connection", could not be added to the ssl exception list anymore (checkbox was greyed out, too).
After a Firefox reset the Yahoo! and other setting changes were gone and back to normal. However I was still unable to save passwords and whenever I restarted Firefox, I was logged out of websites I just logged in (and ticked"Remember me"). All this used to work before.
The final solution to this is to uninstall Firefox, manually delete the Firefox folder (in C:\Users\myusername\App Data\Roaming\Mozilla\) and then install Firefox from the setup exe again. Everything is back to normal now.
Important note: Hands off pdfforge PDFCreator!! It messed around with the Firefox profile and even worse, corrupted it.