Some versions of FF have trouble with pdf files.
I use keyscrambler. It won’t work with later editions. The new version of keyscrambler scrambled my computer, so I had to revert to an older version.
Several extension developers quit developing their extensions, because they couldn’t keep up with the rapid releases.
My main version of FF is 15. I just quit updating extensions and FF.
And there is the ongoing problems between Adobe Flash and FF since about version FF 12 or so.
BTW, what is keyscrambler?