In the autocomplete form element, there is a space before each element when you access it from firefox.

It is not visible because each value has only one line, but in workplace we started adding two lines in some items in the issue WP-3898. Now, we can see that the first line has a space before the text and the second one doesn't.

After checking core, we can actually see that the space is also there.

The solution would be this one:
.form-autocomplete-suggestions li::before

Here you can see how it looks for us in Chrome and Firefox:

Chrome:

Firefox: