[This question is similar to another one (for Ubuntu vs Lubuntu) where there was no real answer other than to look for other Firefox themes -- something that has not worked for me in the past. At any rate, I think this question is different enough. At least I hope so.]
The Widget theme I selected in Lubuntu is Industrial. My understanding is the widget theme dictates what the scrollbar looks like in various programs.
Here's what the scrollbars are supposed to look like under the Industrial theme.
[Annotation should read Scrollbar not Toolbar ;) ]
As you can see, the scrollbars are wide, have a decorated thumb and arrows at both ends.
Below is an image that shows Pcmanfm (on the left) reflecting the Industrial type of scrollbar. Firefox 47.0, on the right, does not. It has a scrollbar that is too narrow for my comfortable use and it doesn't have the arrows at the top and bottom.
I've read that Canonical has branded Firefox, so I thought maybe that was the issue. I went into Firefox about:config and deleted everything I could that said Canonical. Still, I get the narrow scrollbars.
So what I'm trying to do is get the Firefox scrollbars to reflect the Widget theme I selected.
Before anyone asks, yes I've tried the Firefox addon called New Scrollbars (aka NoiaScrollbars). I can widen the scrollbars that way, but I can't add arrows top and bottom and I can't get a decorated thumb. IOW, it just looks too ugly for my tastes.