[Bug 1971502] [NEW] Downloads “Show in folder” button goes to /run/user/100/doc/.../ instead of chosen location

[fantasai]

Public bug reported:

Overview:
Firefox appears to be saving two copies of a downloaded file, one in the chosen location and one in a special /run/user folder, and is showing that folder instead of the chosen location when asked to open the downloaded file.

Steps to Reproduce:
1. Right-click on a page and choose “Save Page As..” or “Save Link As...”
2. Choose a location such as /home/username/.thunderbird/deleteme.html
3. Watch the download icon show download completion
4. Click the download icon in the toolbar, and click “Show in Folder” button in that file's download.
5. Notice that it opens the file in /run/user/1000/doc/RANDOM/deleteme.html instead of where you saved it.

Expected Results:
File is where I asked Firefox to save it, and Show in Folder opens that folder.

Actual Results:
There's an additional copy of the file at /run/user/100/doc/RANDOM/ and that is the folder that gets opened from the Downloads menu.

Additional Notes:
* Tested on Firefox 99.0.1 Mozilla Firefox for Snap Ubuntu cannoical-002 - 1.0 using both my usual profile as well as a fresh new profile, same results both times.
* I've never seen this /run/user stuff before, wondering if it's because of the Snap packaging in Ubuntu 22.04?

** Affects: firefox (Ubuntu)
     Importance: Undecided
         Status: New

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https://bugs.launchpad.net/bugs/1971502

Title:
  Downloads “Show in folder” button goes to /run/user/100/doc/.../
  instead of chosen location

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