Firefox - Can't navigate files and upload to imgur

[Faru]

Hello, new here. New to Linux in general, but loving Mint Cinnamon so far (Cinnamon 20.3, version 5.2.7.).

For an entirely separate issue I'm trying to diagnose for KdenLive on reddit, I'm in Firefox trying to upload a screenshot to Imgur but it's not picking up the picture. Not if I drag & drop or if I try to navigate to the picture location, it's even worse somehow. when I click imgur's "choose photo/video", the file explorer window opens but it's like it locks me into the "Home" tab where only Desktop and Downloads are there- I'm trying to get to pictures and it literally won't change folders. If I click & drag from a separate file explorer window, the site registers there's a file and the little imgur window changes to a night sky, but the file never actually gets there.

What is going on? Why can't I upload?

[Cosmo.]

it locks me into the "Home" tab where only Desktop and Downloads are there

Let us start at this point.

Open the file manager, it's name is Nemo. You should see in your home (at least) 8 folders. Do you confirm? Can you navigate into each of them?
Do you see your user name below the menu bar?

Now start the text editor, it's name is xed. Click to open a file; you should see there the same 8 folders (at least). Again confirmed? Can you navigate to each of them?

[Faru]

Yes, they all appear in Nemo and when I go to open a file in Xed. I can navigate through all of them.

So I guess it's isolated to Firefox... but why?

[Cosmo.]

Next test:
In Firefox: Press ctrl-o. It should open the same window as in the xed example. How does it look for you?

[LoVache]

Could it work with Postimage ?
I'm used to use it by drag & drop.
Maybe some extension like an adblock blocks Javascript ?