STR:
Unknown atm.

This has started showing up in the last week or so.
Nothing seems to be related in the browser console.

This seems related, from the browser console:

SearchService: Failed to load Google from cache: TypeError: can't access property "length", json._urls is undefined
    _initWithJSON resource://gre/modules/SearchEngine.jsm:1824
    _loadEngineFromCache resource://gre/modules/SearchService.jsm:1605
    _loadEnginesFromCache resource://gre/modules/SearchService.jsm:1585
    _loadEngines resource://gre/modules/SearchService.jsm:1005
SearchService.jsm:1608:18

x4 times for the amount of the installed search engines.

In the same session when this happens, the issue doesn't occur in a new window (though it remains in the original window).

(In reply to Itiel from comment #1)

This seems related, from the browser console:

SearchService: Failed to load Google from cache: TypeError: can't access property "length", json._urls is undefined
    _initWithJSON resource://gre/modules/SearchEngine.jsm:1824
    _loadEngineFromCache resource://gre/modules/SearchService.jsm:1605
    _loadEnginesFromCache resource://gre/modules/SearchService.jsm:1585
    _loadEngines resource://gre/modules/SearchService.jsm:1005
SearchService.jsm:1608:18

x4 times for the amount of the installed search engines.

So this may be a separate issue and not related.
On a locally built Firefox I see the same errors logged, while at the same time the placeholder isn't missing the search engine name.

I think the errors in your console are caused by bug 1647256, though they may have gone away already.

Once that is fixed, it'd be useful to know if this still happens or not.

Itiel, can you still reproduce this issue using the latest nightly?

Yes.

I managed to get a solid STR for this:

1. New profile, latest Nightly
2. Restart Nightly and make sure the "Search with Google or enter address" placeholder appears on the urlbar
3. Enter customize mode
4. Exit customize mode
5. The placeholder now reverts to "Search or enter address"

Regression range is 2020-05-12 to 2020-05-13, mozregression can't bisect further:
https://hg.mozilla.org/mozilla-central/pushloghtml?fromchange=78ca58d04a9b107bc6989c3898852160a742e831&tochange=d1114574b777bb00b85f73733a8bd6a78d2ad87e

(In reply to Itiel from comment #7)

Regression range is 2020-05-12 to 2020-05-13, mozregression can't bisect further:
https://hg.mozilla.org/mozilla-central/pushloghtml?fromchange=78ca58d04a9b107bc6989c3898852160a742e831&tochange=d1114574b777bb00b85f73733a8bd6a78d2ad87e

Maybe bug 1631122 ?

Itiel, when this happens, what does the url bar element look like in the browser toolbox's inspector?

(In reply to :Gijs (he/him) from comment #8)

Maybe bug 1631122 ?

I was thinking the same...

Itiel, when this happens, what does the url bar element look like in the browser toolbox's inspector?

Attached screenshot.

The original issue this patch addresses is what happens when exiting customize
mode: the reinsertion of the urlbar (customize mode wraps it in a container)
causes fluent to re-run as there's a DOM mutation, which causes the default
placeholder to be re-translated. This caused issues because we were using
.properties to translate the string that included the engine name, and
fluent for the "default" placeholder string.

This patch fixes the issue by always using fluent, and using a hardcoded
'unknown' placeholder to deal with the case where there is no known engine.

https://hg.mozilla.org/mozilla-central/rev/401ca1af22cd

I've managed to reproduce the issue using Fx79.0a1.
The issue is verified fixed using the latest Fx96.0a1 and Fx95.0b11 on Windows 10 and Ubuntu 20.04. The same placeholder phrase is displayed when exiting the customize mode.