Today I've upgraded all my machines to the latest Firefox 1.5 version. No problem with my notebook and with my 2 machines at work, but today on my home desktop PC I've found a strange problem.
This was the only machine with Firefox 1.0.7 previously installed (the other machines were not updated to this version). After the Firefox 1.5 installation, I've decided to update my previous themes and I've found a problem: installation can't start, on the screen appeared a message bar indicating that "software installation is disabled, click the "Edit Options" button to enable it...".
Ok, maybe the famous "Allow web sites to install software" option is disabled. So I've to go to Tools -> Options -> Web Features -> Allow web sites to install software and enable it. But... where is this option???? Disappeared???? How can I enable software installation??? 
After a little search I've found that it's a signalled Firefox 1.5 problem (bug 310737). On the 1.5 version, the "Allow web sites to install software" option has been removed from the user interface. If you previously disabled this option and then upgraded to Firefox 1.5, the preference setting will be carried over. Since the option to enable software installation no longer exists in the UI, you will need to use the about:config option (URL in the browser window) to reset the "xpinstall.enabled" preference to the default "true" value.
What is curious is that on the other machines where the 1.0.7 version was not installed, the upgrade process was good. However, remember this for the next time... 