hmm... i dont know what to say. thats weird.
i also get a message in - red letters, saying "unsigned" but its allows me to install the extensions..
- are you installing the extensions from the official mozilla website?
here is some info if you are not:
i got it from this website
"You may run into problems if you try to install an extension from anything but the official Mozilla site. That's because the security measures built into FireFox don't allow just any site to install programs on your computer. A good thing I think you'll agree.
However you can get around the problem by telling FireFox the site is OK. To do that select Tools/Options/Web features/ and click on the Allowed Sites button. You can then add the address of the extension download site to the list of authorized sites. Remember to click the "Add" button after you type in the web address.
Another little problem you may encounter is that some extension sites don't offer an "install" option but instead offer a download link. In these cases download the file to your PC. Then, from within FireFox select File/Open file and load the installation file you downloaded. This will install the file. Note that FireFox extension installation files have a file type of .xpi."
Edit: sorry, i searched for your type of problem and i'm kind of assuming the info i posted above might not help
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