The problem is oxygen-gtk is a theme that is inherently bug prone. The bug is not in Firefox but oxygen-gtk.
Oxygen-gtk causes many other subtle issues and I personally think it shouldn't be included in Slackware, or shouldn't be considered more important than key pieces of software when it has a bug, which it will almost certainly have plenty of.
Oxygen-gtk is not conformant to gtk theme standards, it uses various methods that are not really supported to try to get closer in appearance to oxygen than a conformant theme could.
For example this bug report might be related to this problem
Notice the developer doesn't want to try and figure out if there is a bug in glib which his theme is dependent on, and which a good upstream would try. He then says
"Oxygen-GTK, unfortunately, has many such invalid assumptions (but we might not know of their invalidity), which it has to do in order to implement the features we need. It's just a matter of complexity of features we put into it."
This is the politic way of saying, we use undocumented and unsupported "features" to create our theme, if we report this "bug" in glib, it will go nowhere because we aren't using gtk themes properly.
I used oxygen-gtk in xfce in Slackware14 initially because I like some aspects of oxygen, and this theme I liked even better than Oxygen, this was my first time to use it, I never bothered with it when I used KDE or xfce in Arch. I had some weird an not very reproducible glitches that I blamed on Slackware, but then I changed my theme because I got bored, and had no such problems, then went back to oxygen-gtk and again had subtle issues.
I personally think oxygen-gtk shouldn't be included in Slackware, and should be in extras or a simple Slackbuild. I think it is very bad idea to downgrade one of the more popular and important pieces of software to a version older than Slackware 14 shipped with because we don't want to delete a stupid theme, especially a theme that is inherently buggy because it is non-conformant to the themeing rules.
Some people want the ESR version of Firefox, that seems a separate issue, as I'm sure just as many of us want the latest release. I think it would be nice for both to somehow be available.