Thanks for sticking with me so far CM.
Yep, I have the box labelled "Enable Software Installations" checked in Mozilla's Preferences > Advanced > Software Installations.
Maybe I should be more specific: the .xpi I'm trying to install inparticular is the "Home Button" for Mozilla on my personal work laptop. Had no problems with .xpi files on my desktop at home--they zip right along at home. But for some reason, they just won't work on the laptop. I tried one other .xpi on the laptop, and it didn't go, either. Maybe I'm trying bum .xpi files.
Funny that I can't find anything in Mozilla's help, but I'm not giving up on this, not so long as there's somebody else out there who feels my frustration: http://www.linuxquestions.org/questi...ight=XPInstall