User Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.16; rv:78.0) Gecko/20100101 Firefox/78.0

Steps to reproduce:

Select "File --> Open File..." from menu.

Actual results:

The file-open-dialog and the file-save-page-as-dialog open in Spanish instead of English. Although all menus and other text is correctly shown in English, it seems that system dialogs have the wrong language.
All buttons of the file dialog are named in Spanish instead of English (opciones, cancellar, abrir).
The Favorites' headline is named "Favoritos".
System provided folders are named in Spanish too (e.g. "Downloads" --> "Descargas").

Additional info: I'm located on Tenerife (Spain). My Macbook is set to German language. Firefox is set to English language.

Expected results:

The file-open-dialog should open in English, because Firefox is set to English language.

I believe the intended behavior is to show the "Open File..." modal in the language of the operating system, not the browser's set language, but I will somehow get this checked.

Considering the fact that your modal is displayed in Spanish, not the Operating System's set language (German) and neither the browser's set language (English), this is a little more complicated because I'm not sure what could influence it.

Would you please:

1. Attempt to reproduce your issue while the browser is opened in safe mode and tell us the result?
   (https://support.mozilla.org/en-US/kb/troubleshoot-extensions-themes-to-fix-problems)
2. Provide us the information displayed inside the "about:support" page of the affected browser.
3. Check the language selected in the System Preferences / Language & Region / General tab; I suppose it's German.
4. Check if there some specific language selected in the System Preferences / Language & Region / Apps tab, for your browser specifically.
5. Are you using Firefox ESR v78.8.0esr on Mac OS 10.16 when this issue reproduces?

To the DEV picking this up: I have chosen the (Core) Internationalization: Localization component because I could not pick any other. If incorrect, please change it to a more appropriate component and more importantly, can you confirm the statement in the first paragraph of this comment?

Answers to your questions:

1. In safe mode it's the same issue, it's in Spanish.
2. I've pasted it above, I hope the format is ok, I took "raw format".
3. Correct, it's German.
4. No, nothing, it's empty.
5. I'm using Firefox 78.8.0esr (64-bit) on macOS Big Sur 11.2.3

I've set the Firefox UI language to Spanish once some time ago, but then switched back to English.
That's why English and Spanish are selectable in about:preferences -> Language and Appearance -> Language.
See attached screenshots of firefox preferences.

In about:support I wonder why "Default Locale" is set to "es-ES"?

You're using an es-ES build, with English (en-GB) installed on top as a language pack.

You might want to replace that build with an en-GB build
https://www.mozilla.org/en-GB/firefox/all/#product-desktop-esr

Francesco, you got it!! Thank you very much!
I've downloaded and installed the en-GB build, now all system dialogs opened by FireFox are in English.

What is confusing for me and maybe for other users too:
Why is the language of system dialogs, opened by FireFox, depending on the build language instead of the selected UI-language?
Imagine installing Firefox with e.g. chinese build language, then an english speaker (who doesn't know Chinese) would want to use this FireFox. The Chinese would install an english language pack for the english user and activates it. As soon as the english user saves or loads anything or uses any other system dialog, he can only see chinese letters on the buttons etc and would be lost.

I would like to make a feature request, where the language of system dialogs is depending on the activated language pack first. If no other language pack is available, then the build language shall be used.

I can confirm this issue on Release v88.0.1 and ESR v78.10.1esr. This issue also occurs with other combinations of languages. The incorrect language is shown in the OS type modules opened with the "Open File...", "Save Page As...", "Page Setup...", "Print..." buttons from the File menu.
This issue does not occur in Windows 10 and cannot be reproduced in Nightly builds.
Considering the reproduction on the latest ESR build, this is not a recent regression. NI me if further investigation is needed.

Thank you for the report, Pink Ink!