Time `<select>` not working in Firefox

[Malvoz]

Repro steps:

1. Open https://maps4html.org/experiments/datacube/ in Firefox
2. Open the time setting in the layer control
3. Click the <select> and mouse over the options
4. <select> closes

I thought I fixed this in #492, apparently that only fixed the issue in Chrome, weird, I usually try changes in more than 1 browser. Anyhow, this is not really my area, perhaps @ahmadayubi can take a look?