I found this on ISCA: 


Dec 1, 2005 05:08 from Green



I downloaded Firefox 1.5 for Windows. I removed my old version of Firefox using

the Add or Remove Programs feature from the Control Panel. I deleted the old

installation directory. I installed the new Firefox version. I went shopping

for extensions. I was unable to install any extensions and received a message,

"Software installation is currently disabled. Click Edit Options... to enable

it and try again." Trouble was, upon clicking the Edit Options button, a dialog

box from Firefox Options opens, the same one that formerly had an option to

enable or disable software installation, but that option is no longer present.

Instead, there is a "Warn me when web sites try to install extensions or

themes" option to be checked or unchecked. After Googling a bit, I found where

someone named Peter R. Cook posted this workaround:



"I think you have fallen foul of a problem I found in the beta 2

release. If you upgrade from V1.0.x , and the existing configuration has

software installation disabled the new version appears to have no way re-enable

installation. I fixed the problem by loading about:config (type about:config

into a blank window) and guessed that xpinstall.enabled was the problem. In my

installations it was set to "false". Editing it manually to 'true' has allowed

extensions to install."



Perhaps it's just me, but I would think Mozilla would have kept that option in

the normal options list. Anyway, I thought I'd post it here in case anyone else

finds him or her self in the same strait as was I.