I like using Firefox's pdf viewer but there on thing that bother's me about the interaction between the order collapsing the sidebar and setting the view to fit to width.
If you collapse the side bar and then fit to width the document is fit to width and that is fine.
However, in th other order if you first fit to width than collapse the sidebar then the document no longer it fitted to the width and reclicking the fit to width setting does nothing.
You have to set the viewer to another setting like fit page and then redo the fit to width.
It is not a big deal, but just irksome.