The scrollbar in Firefox has been broken for Linux desktop users for a very long time. By default, the scrollbar is the size of a toothpick and a nightmare to grab on large screens without clicking something else.
You can make the scrollbar *slightly* wider by going into the settings and enabling "always show scrollbars", but that just makes it two toothpicks wide instead of one.
If you go into the about:config menu and search for "widget.n", you'll get a small selection of non-native theme settings.
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widget.non-native-theme.scrollbar.size.override
Will override the width of the scrollbar. Changing it to 12-16 gives a scrollbar of a decent size to work with on large monitors.
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widget.non-native-theme.scrollbar.style
Set this to 2 to use GTK themes, which will tell Firefox to use your currently selected system theme. Your theme needs to support GTK3 or you may encounter strange behavior.
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widget.non-native-theme.gtk.scrollbar.allow-buttons
This is my favorite, it enables the scrollbar arrows. You'll need a GTK theme which also has the scrollbar arrows, which are getting scarce these days. My Radiance-Fedora theme supports this and should show scrollbar arrows.