Recently I posted a question in the Mozilla forum like this:

I use Firefox 47 with LinuxMint 17.3 KDE desktop
In preferences I have set Downloads: always ask Whenever I click on a download
Whenever I click to download item I am presented with a window to navigate my filing system. Inside this window my mouse behaves erratic. The doubleclick interval is much shorter than in any other application on my system Clicking or doubleclicking on any item in the list of direcories on the left side produces no result Clicking on cancel or save does nothing on the first click, only on a second
I am at a complete loss how to navigate this window

I made a screenshot of the window to show what I am talking about but I am at a loss how to insert it here

One member of the Mozilla Forum answered as follows
Hi

I have recently seen a similar issue for another user of a KDE desktop environment. This is a bug that is listed at Ubuntu and is known about for distro builds of Firefox using some KDE desktop themes.

There is a possible solution, but I would strongly recommend that you get in contact with a dedicated KDE support forum who will be better placed to help