I have never seen an extension where the author was verified, so yes it should be considered safe. The addons system is set up to scan for exploits during the upload process to the server, plus once an addon reaches a specific download level the code is visually verified.

Thank you for assuring me it was safe to load the "Menu Icons Plus" Add-on. However, when i clicked to install it the same message about preventing it came up:

  "Firefox prevented this site (en-us.add-ons.mozilla.com) from asking you to install software on your computer."
  Why can't I install it, what is wrong with it? Is there another app that will let me change the icons in the drop-down Bookmarks list?

You can click the allow button on that pop-up and follow further instructions install the extension and to restart Firefox to finish the installation.

It is possible that you lack the default permission to install software from the Add-ons site.

• Tools > Options > Security : "Warn me when sites try to install add-ons": Exceptions

I finally was able to install the Menu Icon Plus Add-ons, or so it appeared because a window with a progress bar saying it was installing it came up. But when I followed the next instructions (shown in the "Uploaded images" window below and selected the Menu Icons Plus (from the Add-ons Manager), a window appeared with "Menu Icons Plus 2.4" in it but there was no "Options button."

  I then clicked on the Preferences box at the lower left , which brought up a Menu Items window but I couldn't get it to do anything or set different Preferences. (I also had already restarted Firefox as instructed, to no avail.) What do I do now to use the Add-ons?