If I open more than 9 tabs (10 or 11) the tab scroll bar covers more than half of the tabs making them unreadable. This appears with the default firefox theme and with at least 4 other themes.
The scroll bar should appear bellow the tabs and not cover them.
Also using Bookmarks Scrollbar 1.0.5; however, tabs are allowed 2 rows. The 2nd row covers almost 1/2 of the 1st row.  Haven't yet tried removing the extension to see if it changes things since I seldom reach a 2nd tab row.  If the extension is the cause hopefully the author will soon have it corrected -- although it's been 5 months since reported here.