I just found my USB drive with PortableApps on it and decided I needed to update Portable Firefox. I also decided to update all the add-ons and add a couple of new ones.
Thanks to the FAQ, I downloaded the latest version (3.0.10) and installed it using the PortableApps menu. The program runs fine, and when I searched for updates to the add-ons it downloaded them and ran through the usual installation steps. One didn't update because of a file error, but I'll get that later. I also added Tabs Mix Plux and Xmarks.
Now I've restarted Firefox Portable several times, but all the add-ons are showing the message, "This add-on will be updated when Firefox is restarted."
I don't know what I should try next.