This issue has already been resolved, though I am not sure if you will get this type of problem every time Firefox will update itself to its latest version, F-Secure should also update.
I've seen similar reports.
Yes, the Browsing Protection addon is still causing massive performance problems.
Hope they will sort this out with 2013 as they move into the new approach with no browser addons.
Until then, you can disable browsing protection if youd rather have performance over security
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