I'm afraid I don't have an answer, but I can confirm that it doesn't work for me either.
If it helps the product team, I'm using Firefox Quantum v62, and I also tried with Firefox in Safe Mode, with all add-ons disabled.
Could you please share a bit more about your issue?
The user @Hugh1 has faced same issue with you, please check the response within the following thread:
The response from PowerApps staff @miglisic as below:
Indeed, Firefox 62.0 introduced a change which broke how Date Picker handles its flyout. A fix has been completed and will roll out the week of Sep 24. I'm afraid there is no known workaround, other than using a different browser or different version of Firefox.
Please take a try to run your app within other version of Firefox (not the 62.0 version), then check if the issue is solved.
If the issue still exists, please take a try to run your app with other different browsers.
An employee had issues with the calendar popout, but it is still possible to type the date. I tested in version 62.0.2 to confirm their observations. The Date Picker works fine in the rest of the browsers. I read through the other thread, it sounds like a fix was supposed to be released 24 Sep but that does not seem to be the case. Nobody there has confirmed it working in a 62+ version of FF.
Rolling back browsers isn't an option. Hopefully, the fix will come soon.
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