Please see Re-enable add-ons that were disabled when updating Firefox .

The best bet would be to contact the add-on developers and let them know you'd like to use their extensions on the new version. You can use Add-on Compatibility Reporter to do this.

You can get the previous version at http://www.mozilla.com/en-US/firefox/all-older.html

Thank you for your answer. Hopefully, the F-Secure team will soon develop an upgrade of this extension. However, for now it seems that the browsing security is compromised, since there is no evidence that this F-Secure feature is functional with FireFox 4. Perhaps the FireFox team should add this in the description of the known issues about this version.

for a certain reason the F-Secure toolbar suddenly works with Firefox 4 on my PC. I donk't now why, but it suddenly works today. the last days i've had the same problem as alex45. Maybe it now functions with you new firefox 4 browser, too. It's very strange, because there was not an update for the new browser, at least I've had not noticed any updates. Maybe it's F-Secure with its automatically update?

Yep! That' right, checker. The F-Secure Add-on reports that it was updated on Monday 28. Problem solved.