As kjk1281 mentioned, first try clearing your cache and cookies. To do that in Firefox 3.6, Click Tools in the main menu and select "Clear Recent History." Make sure you select both your cache and cookies to clear. If you're still having issues, please post again with any URLs which are giving you errors along with your OS.
odigg, if you could elaborate on any editor problems you're seeing, that would be helpful (a screen shot of any problem would be great).