Right now I am playing with Firefox 45.9.0 GA, (2017-10-04). But it does this with all versions of Firefox which leads me to believe that something I did or changed two weeks ago is interferring with it loading. But I'll be damned if I can find whatever that is.
It took me a while to do it, but I did manage to get the base install of YUM/RPM to actually work beyond the initial install and update; like most people using OS/2 or some variant, I had my own way of handling *.dlls; I really had no choice but to conform to the YUM/RPM method of doing things to get some programs working and cooperating.
Right now, Arca Noae YUM install works fine, (though it took quite a bit of effort to get there), and indicates all is loving and caring in the world... of OS/2 hehe, (certainly not on the global scene where Empire-buidling is rampant and roaring under the obvious guise of "spreading democracy" & "peacekeeping"). So, it seems likely that all DLL requirements are met for the version that I referenced above. I did follow the RPM_REQUIREMENTS file included with the package and until just recently, it wouldn't even attempt to load but only produced a XUL.DLL error popup window; but fixing YUM fixed that.
Arca Noae's YUM package checks for duplicates and olders versions when installing a dll(s) so that should not be an issue here... though I am not sure that I can rule that out entirely. I think that the important clue is that it affects all variants of Firefox. However, I had no idea that there was anything that could do that yet, not affect anything else on my system; it's a real puzzler, it has even stopped the ancient and early version of Firefox from running, even from 2011. Clearly something is in conflict with something else or somehow, something has been inadvertantly deleted. But how would I even begin to check if some obscure base OS file/dll has been deleted? Right now, I do not have another working version of OS/2 on this machine to compare it with. If all else fails, that will be my next goal and I will work from there... but right now that's a lot of work when time is tight. I actually had to lower my standards and use Windoze to type this post; my, how times have changed.
I will check out that site David. Maybe I can find a clue there.