Based on what you told me, I decided to try fixing the problem through css (using the "stylish" extension) and it worked!
I used the following code to fix it, where the values of min/max-height and min/max-width are the correct dimensions of the scrollbar:
scrollbar[orient="vertical"], scrollbar[orient="vertical"] thumb, scrollbar[orient="vertical"] scrollbarbutton
{
min-width: 9px !important;
max-width: 9px !important;
}
scrollbar[orient="horizontal"], scrollbar[orient="horizontal"] thumb, scrollbar[orient="horizontal"] scrollbarbutton
{
min-height: 9px ! important;
max-height: 9px !important;
}
Thanks again for your help!