Sub category.Can't choose anything in Parent, no list at all

I've made a clean installation of 2.3.0.2. I have an annoying problem with sub-categories I hope somebody can help me with.

I've created a main category: Clothes
It works fine and it displays as a top-category as I want.

Then I create a category called Shorts
I want Shorts to be a subcategory under Clothes. That is Clothes --- Shorts.
I go to Shorts category, Data tab and Parent.

When I'm in Parent no list at all appears. If I start writing the category Clothes, it doesn't appear as a suggestion below Parent. I can't choose Clothes from the list, as there are no list appearing.

If I write Clothes in Parent and save it doesn't end up as a sub category to Clothes. Also when I open Shorts category again the Parent/Clothes is not there.

What's the problem?

I just noticed a weird thing. I always use Firefox. With Firefox I have the problem with no Parent list.
When I use Exporer the list appear as it should.

I hate using Explorer. Do you have any ideas how I can get it to work with Firefox?

Adding information.

On my laptop Firefox has no problem with autocompleting Parent list.
On my desktop Explorer has no problem with autocompleting Parent list.
On my desktop Firefox has problem with autocompleting Parent list.

What could be the explaination I don't get it to work on Firefox on the desktop?

P.S When I used OC 1.5.5.1 the autocomplete worked fine on Firefox on my desktop.

Problem solved.

Maybe somebody else find himself with the same problem. To solve it I had to remove(inactivate) a Firefox Plugin called RightToClick. Now everything works fine.

Glad thats working now and well done on posting up the solution....will help others in similar situation

We use FireFox as well but do not have that plugin activated or installed. Just by chance we stumbled upon why sometimes it was working and other times it would not. If we did a 'long' left click with our mouse it would not select the highlighted category. HOWEVER if we did a 'fast' aka quick left click with our mouse it would insert the category into the selection box and allow us to save it.

Perhaps its related to a setting on our computer for mouse clicks which relates to what FireFox expects - this might explain in certain cases why one can choose on a phone or tablet but not a PC. Its due to our mouse click settings? Frustrating but like video games sometimes you need to try the un-obvious to get the tool LOL

LarsSven.....Thank you soooo much! I was struggling with this for days. I couldn't figure out why it worked sometimes but not most times. It was the click speed just as you described. Thanks again for posting this.