Except ... after installing Petra and then downloading a file using Firefox, I right-clicked the file's entry in Firefox's download list and clicked "Open Containing Folder", expecting to, well, open the Downloads folder. Instead, Firefox launched the Disk Usage Analyzer (?!).
A web search found a small number of reports of this with no fixes posted. It took a lot of digging before I finally put together the facts that (a) Firefox uses MIME types to decide what applications to launch and (b) the name of the disk usage analyzer is "Baobab", not "Disk Usage Analyzer". Armed with that, I found a line in /usr/share/applications/mimeinfo.cache (line 368 in my copy; your mileage may vary) that read:
inode/directory=baobab.desktop;nemo.desktop;So I ran sudo gedit /usr/share/applications/mimeinfo.cache, changed that line to
inode/directory=nemo.desktop;baobab.desktop;and saved the file. Immediately (without having to restart it), Firefox sobered up and starting opening the Downloads folder in Nemo. I guess Firefox uses the first available program in a list of applications for a given MIME type, and for whatever reason someone thought Baobab should have precedence over Nemo.
Update: As soon as I posted this, I went back to fiddling with my installation, and installed the nemo-dropbox package from Synaptic, which integrates Dropbox with Nemo. It also downloaded Dropbox, regardless of the fact that Dropbox was already installed. :-( Not only that, it apparently modified mimeinfo.cache, because mimeinfo.cache reverted to its original self. I had to edit it again. Looks like this may be the gift that keeps on giving. :-(
One other note to self: the annotations on the Dropbox folder (up-to-date, being synched, ...) that are the whole point of the nemo-dropbox package only show up after you stop Nemo (nemo -q) and then restart it.
Update #2: I think it's solved now. After noting that /usr/share/applications/mimeinfo.cache had reverted yet again, I added the line
inode/directory=nemo.desktop;baobab.desktop;to ~/.local/share/applications/mimeapps.list. That should override the entry in /usr/share/applications/mimeinfo.cache.