Bugzilla

STR:
1. Set another browser as your system default browser.
2. Start Firefox Nightly with a fresh profile, and then immediately quit.
```
mkdir /tmp/test-profile
firefox -profile /tmp/test-profile
[Ctrl+Q]
```
3. Start Firefox Nightly again, now immediately visiting some foreign-language page.
```
firefox -profile /tmp/test-profile "https://es.wikipedia.org/wiki/Wikipedia:Portada"

ACTUAL RESULTS:
* Two modal dialogs appear: (1) a translation dialog appears, (2) a default-browser dialog appears underneath it.
* The translation dialog appears in the foreground, but in fact it's not interactive.
* The default-browser dialog appears in the background, but it *is* interactive -- and your clicks in covered-up parts of that dialog will make it through to buttons/UI that you cannot see!!  This is particularly noticeable if you hover/click the (covered-up) label for the checkbox in the background dialog.

EXPECTED RESULTS:
* We shouldn't show overlapping modal dialogs/notifications.
* Or, if we do show overlapping modal dialogs, we should be sure that their visual stacking order matches their interactive stacking order.

(There might be a Layout or Web Painting bug hiding here, with the paint-vs-click-event stacking ordering mismatch; CC'ing emilio & tnikkel & myself for potential thoughts/investigation there.  But also: independent of that, we may want to prevent ourselves from getting into this multiple-modal-popups-at-once situation entirely, maybe, via some Messaging System logic/checks?)

STR:
1. Set another browser as your system default browser.
2. Start Firefox Nightly with a fresh profile, and then immediately quit.
```
mkdir /tmp/test-profile
firefox -profile /tmp/test-profile
[Ctrl+Q]
```
3. Start Firefox Nightly again, now immediately visiting some foreign-language page.
```
firefox -profile /tmp/test-profile "https://es.wikipedia.org/wiki/Wikipedia:Portada"
```

ACTUAL RESULTS:
* Two modal dialogs appear: (1) a translation dialog in the foreground, (2) a default-browser dialog, in the background.
* Even though the translation dialog appears in the foreground, it's not interactive. (Its buttons have no effect.)
* Even though the default-browser dialog appears in the background, it *is* interactive -- and your clicks in covered-up parts of that dialog will make it through to buttons/UI that you cannot see!!  This is particularly noticeable if you hover/click the (covered-up) label for the checkbox in the background dialog, since that part is partially-visible.

EXPECTED RESULTS:
* We shouldn't show overlapping modal dialogs/notifications.
* Or, if we do show overlapping modal dialogs, we should be sure that their visual stacking order matches their interactive stacking order.

(There might be a Layout or Web Painting bug hiding here, with the paint-vs-click-event stacking ordering mismatch; CC'ing emilio & tnikkel & myself for potential thoughts/investigation there.  But also: independent of that, we may want to prevent ourselves from getting into this multiple-modal-popups-at-once situation entirely, maybe, via some Messaging System logic/checks?)