I can't see where I am in the pages since the color of the scrollbar is the same as the background ...
Can I change the color, maybe with ./gtk-2/apps/ff.rc ? I couldn't find anything about it.
My theme is Ambiance and I added
in ~/.theme/copy_of_the_Ambiance_theme/gtk-2.0/gtkrc after tweaking the shades and it worked.
Does this problem exist on all websites? It could be that a page is handling scroll bars within their code, or has overflow disabled in their html.
Do you have a theme applied? Check Tools > Add-ons > Appearance and switch to the default theme to double-check that this isn't the problem.
Do you have an extension (like NewScrollbars) or plug-in that might be altering the way Firefox scrollbars are handled? If so, disable them and try again.
If it is none of the above, Firefox uses the current desktop theme for its colors (including the scroll bar). Try switching your current desktop theme to the default--or at least to something different--to see if the problem is corrected.
Firefox 44.0.2 / Ubuntu 14.04 / NoiaScrollbars 1.2.4
The easiest way for noobs like me is to install an add-on for it.
NewScrollbars aka NoiaScrollbars does the trick.