Symptoms
The problem relates to Firefox version 42 and above. If the file pciker is used to select a file, usually to upload it, Firefox crashes with a segmentation fault. This happens on Linux with KDE4 as the DE, and only effects some machines, not others.
Diagnosis
The problem occurs when using the GTK theme oxygen-gtk2. It must be a bug in the GUI shading code. It only effects some machines, likely due to differences in OpenGL driver behaviours between different graphics cards and/or Xorg setup. The build option for oxygen-gtk2 which causes the crashes is called ENABLE_INNER_SHADOWS_HACK. When set to 0, the crashes no longer occur.
Prognosis
There are four distinct possibilities to solve this problem. They are as follows:
1.) Use a different GTK2 theme. This is set in the file ~/.gtkrc-2.0 or in the KDE System Settings->Application Appearance->Gtk Configuration.
2.) Don't use a theme for GTK2. This can be done by removing ~/.gtkrc-2.0.
3.) Build and install oxygen-gtk with ENABLE_INNER_SHADOWS_HACK set to 0.
4.) Use an older version of Firefox, v41 or below.
My personal favourite and recommendation is option 3. Since it offers the least compromise.
Slackware 14.1 solution
The following SlackBuild fixes the issue. Download the following file and follow instructions below.
https://blog.potensol.com/media/uploads/oxygen-gtk2.tar.xz
1.) Extract the tarball
tar xvf oxygen-gtk2.tar.xz
2.) cd into the SlackBuild directory
cd oxygen-gtk2
3.) su to root
su
4.) Build the SlackBuild
./oxygen-gtk2.SlackBuild
5.) Install the package
upgradepkg /tmp/oxygen-gtk2-1.4.6-x86_64-1_FDo.tgz
That's all folks!