I'm using Firefox 1.5 as well and receive a completely different message: ''To protect your computer, Firefox prevented this site (dictionarysearch.mozdev.org) from installing software on your computer.'' Tha comes up in a yellow bar at the top of the screen, and clicking the Edit Options button to the right brings up a box titled ''Allowed Sites - Software Installation.'' There is shows a list of allowed sites, then gives me the option of allowing dictionarysearch.mozdev.org, as well as deleting one or all of the allowed sites.
My only suggestion at the time would be to go tools->options, select the Content tab, and temporarilly uncheck ''warn me when websites try to install extensions or themes.'' With any luck you will then be able to proceed. If not, could you grab a screenshot or two of what you're seeing? (Use a free program such as ScreenHunter to take the screenshot, then upload it to the web using ImageShack and post the link here.) Hopefully seeing what's coming up will help in figuring out why and how to fix it.
John
I have Firefox 1.5 (great) and I'm trying to install some extensions from the website. For instance, DictionarySearch at http://dictionarysearch.mozdev.org/installation.html
Whenever I try to install, and this happens on the mozilla update site as well - ex. https://addons.mozilla.org/extensions/moreinfo.php?id=35&application=firefox - the little notification bar-thing below the tabs pops up and says "Software installation is currently disabled. Click Edit Options... to enable it and try again." Ha. So I click on the Edit Options button which helpfully appears to the right of the message, and there's a checkbox asking whether I want to be warned when sites try to install software. Ok, the box is checked, and there's an 'exceptions' button for adding websites that are allowed by the user to install. It says put in the exact web address for the site, so I copy and paste. Hit OK, OK. Retry install. Same thing. Ok, I'll UNCHECK the box. Nope. ARGH!! I NEED THESE EXTENSIONS!!! HELP!!!

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