how do I change the "do this automatically" when the box is greyed out
When I download .avi files and the like, Firefox assumes I want to "open" them, forcing me to click the "Save as" button every time.
The "Do this automatically from now on..." box I want to use, is greyed out; in other words, I can't change the default setting.
I tried to alter the setting in the tools -> application to "Save file", but it didn't help me.
See: "File handling in Firefox 3 and SeaMonkey 2"
Not that it's related to the problem you're having, but saying it since it's still important: you should definitely upgrade to Firefox 3.6 asap, as 3.0.x is no longer supported by Mozilla. This means that you're no longer patched for security holes and are exposed to online threats like viruses, etc.
Upgrading is easy - just go to mozilla.com and download the latest and greatest version of Firefox. Good luck!