slider size too small

[Paul Cartwright]

I am running fedora 22 I686 on a laptop, an older Dell model.. I 
remember having this issue before ( and way back I knew where to look to 
fix this in Windows)..
in Thunderbird the scroll slider bar is very tiny, maybe only 3-4 pixels 
wide. Hard to click in that area & hard to grab the slider to scroll 
down. How do you make it larger?? I hate to mess with themes, they all 
seem too wild for me.. is there a setting somewhere?? I am running Mate 
desktop, if it matters.. the slider in Firefox is also small, so it 
isn't just Thunderbird..