I think it's kinda awkward that mozilla remove this useful thing at the new version of Firefox 3.6. It made me tried to search for explanations about "where is properties at firefox 3.6?" Maybe Mozilla Firefox developers might not realize, one of the reason why i choose firefox for my default browser? One of the reason is because firefox have that properties. Me as a web designer use this feature most of the time when i need to find out what is the size for pictures. Safari don't have it, also Google Chrome. Well since i'm not a new comers in internet media, so i could search for help related to this terrible issue, Firefox choose to remove properties from it's default and put it on addon, so user have to install it by them self before back in using it (properties). But you might kick many firefox users without you realize it. There is also people who don't want to think and search too much about "where is properties at firefox 3.6" at Google.

For you that might need to use this feature back to your firefox new version, you might want to try this mozilla firefox properties addon.