The latest Greek macOS Nightly from https://www.mozilla.org/en-US/firefox/nightly/all/ is broken. I see a brief flash of a browser window with an XML parsing error before it closes without any console output. Bug 1539759 has also been mentioned as another potential cause for this issue.
Bug 1548990 Comment 9 Edit History
Note: The actual edited comment in the bug view page will always show the original commenter’s name and original timestamp.
The latest Greek macOS Nightly from https://www.mozilla.org/en-US/firefox/nightly/all/ is broken. I see a brief flash of a browser window with an XML parsing error before it closes without any console output.