You may have corrupt xulstore.json or Localstore.rdf file.

Close Firefox. Open your file browser to this hidden location:

• Mac: *~/Library/Application Support/Firefox/
• Linux: *~/.mozilla/firefox/
• Windows: %AppData%\Mozilla\Firefox\
• *C:\Users\<user>\AppData\Roaming\Mozilla\Firefox\

Locate the above files. Then rename or delete them. Restart Firefox.

I followed the procedure that you suggested, but it didn't work .

Start Firefox in Safe Mode {web Link} by holding down the <Shift>
(Mac=Options) key, and then starting Firefox. A small dialog should appear. Click Start In Safe Mode (not Refresh).

Is the problem still there?

After your original post, another Sierra user reported this same problem when using two screens with different resolutions. Does this match your experience:

Download window too small, can't be expanded (unsolved)

Yes, I, too, have multiple monitors, with differing resolutions ( I used a Thunderbolt-to-full-size-DVI adapter to hook-up an old Apple 2005 Cinema Display, via my Thunderbolt 2 port ) .

Firefox's windows have never reliably restored themselves in the correct position on my multiple monitors, when Firefox is brought-up .

Would I lose my Firefox's "history", if I were to delete its main ".plist" Preferences file, like suggested in the other article ?

I'm not a fan of multiple monitors for many reasons, but the confusion about where things open is a big part of it.

The .plist file stores Mac-specific settings. Firefox stores its history in the places.sqlite file in your profile folder, near where you removed xulstore.json. Removing the .plist file should not affect places.sqlite.

O.K. -- I deleted the .plist file, but it hasn't recreated it, yet ! There was also a : org.mozilla.firefox.plist.lockfile , which I didn't touch . It may have solved the problem .

Here is the diagnosis:

If you move the Firefox window, that requests the download, to the original Retina's screen, you get a normal pop-up window, for the download selection dialog .

However, if you move the Firefox window to an alternate resolution, and request the download, you get a shrunken pop-up window, for the download selection dialog !

This is STILL not fixed, under macOS Sierra v10.12.2, and Firefox v50.1.0 !

I'm using a mid-2015_15-inch_Retina_MacBook-Pro .

I'm using a Thunderbolt-2/Thunderbolt-1_to_Full-size-DVI_adapter, to connect my 2005-Apple-Cinema-display, as a secondary monitor .

The secondary monitor is virtually, off-to-the-right, of my MacBook_Pro's Retina's laptop screen .

rsblanchard

Please check your Inbox at the top of the page for the PM I just sent to you concerning this issue.