Yeah if it's listed on the Firefox website its more than likely that it would be safe to use. Just to be sure make sure that your anti-virus protection is running and up-to-date.
The only one that i think you shouldnt use is fasterfox or something like that b/c it modifies firefox's registry (just like regedit) and most changes are worse than what you can do manually. Other than that I think they should all be safe. Just make sure to check your processes running in task manager every once in a while to make sure something extra wasnt installed. I got that once with a vista sidebar gadget. Not good!!
agreed. don't mess with the ff registry. i did. ended up destroying the old ff--- wouldn't update and FEBE (an ff addon that clones saved bookmarks, cookies, passwords, etc) didn't work when i wanted to clone all my stuff to the new build