how do i let firefox allow extensions to be installed?

I just upgraded to 1.5. in the old one there was a check box for "allow web sites to install software." now there is not, and it won't let me install my favorite extensions like forecast fox, etc.

could someone tell me what clicks i need to make to allow the plug-ins to install?
I have the windows xp version, but i assume other versions have the same menus.

thanks!

Click on Options and it's under the web featues Icon

Click on Options and it's under the web features Icon

i don't see it .

okay found it, it's not in the options!

you open "about:config" in the address bar and all the way at the bottom is xp.somethingorother. it is bold. you double click it to change it to true, then you are okay. just remember to set it back.