The image didn't upload with the question.

There's a similar report from last week (unsolved): Why, on macOS Sierra, is the download-window-popup, so small, that I can't select options, within-it ?

If you press the tab key while that window is displayed, does the content shift around in the window or does the cursor/focus just disappear into the invisible areas?

I know very little about Macs, but have seen this advice over the years for problems with the parts of Firefox/Mac that are under system control:

Try to remove the Firefox plist file (org.mozilla.firefox.plist).

• http://www.macfixit.com/article.php?story=20060606082246983 How .plist files become corrupt and troubleshooting the results (alternate link: https://www.cnet.com/news/preferences-files-the-complete-story-part-v-how-plist-files-become-corrupt-and-troubleshooting-the-results/)

Go to "~/Library/Preferences" and remove the plist file for Firefox (org.mozilla.firefox.plist)

Not sure whether that will help with this dialog.

Thanks @jscher2000 This appears to have worked, but I need to check again when I'm plugged into my external monitors to replicate the environment I had last night.

I did not think to unplug my additional monitors when I had the issue... doh. I'm chalking it up to frustration.

For anyone else who has this problem, I removed all the the plist files with "mozilla" or "internet" in them.

I'll report back when I get home to my "command center."

Oh... if you cause the screen resolution to change while Firefox is running, that may indeed confuse Firefox about font and bar/dialog sizes. It has been reported by users with higher-than-full-HD displays switching from an app that somehow runs in full HD back to the higher resolution desktop. But I don't know if that is related.

OK problem is still here now that I've plugged into my additional monitors.

I use the laptop screen plus two 23" inch monitors. The laptop screen is set to 1440 by 900. One monitor is connected directly to the HDMI port and set to 1080p resolution. The other is DVI connected via a Thunderbolt adapter set to 1920 X 1080.

When I open Firefox on the laptop screen is appears like the image below. when I drag it up the HDMI connected monitor is readjusts and looks ok. No matter what monitor Firefox is on, the download prompt always pops up on the laptop screen and it too small to show the action buttons.

Does this sound like symptoms of your last post? I can go to Mac for Apple support, but I only seem to be having an issue with Firefox (that I've noticed).

Is there a way to obliterate all Firefox related preferences and "config files" from my machine and then reinstall it? Would that even help? Just downloading and dragging back to my Application folder doesn't really touch preferences.

I use Firefox to do my work and I frequently have to download files, so this is quite painful to me right now. I really don't want to have to switch to Safari or Chrome in order to do work tasks.

I personally do not use a multi-monitor setup and when I watch people working, I'm always puzzled how things open on a different screen than the one you would want. Shouldn't there be something in the environment that forces everything for a particular app to a specific screen?

Anyway, I wouldn't do anything drastic because if Firefox thinks your laptop is your main screen for dialogs, that's probably a built-in behavior and not just a saved setting (we've already cleared the files related to saved display settings).

OK I'll keep poking around and try to keep this thread updated if I ultimate find a solution. I really appreciate your help @jscher2000!

I also noticed the image I referenced in my last message again was not uploaded. I attached it here in case anyone happens across this thread and needs the context.

Eventually, I felt that I used up too much productive work time troubleshooting and working around this issue so I used a shotgun to kill a fly and restored my Mac to factory settings (on Sierra).

Ultimately, this fixed the download prompt issue, but I would be curious to know if anyone else eventually solves this without a restore.

Yeah -- you move the Firefox window, to the original Retina's screen, and THEN request the download, and you should get a normal pop-up download-selection-window (I know -- this is a workaround) .

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 .