I ran across a problem in Firefox 3.6.16 where my JavaScript applications produced with ExtJS were producing menu separators that were extra thick.
Even Ext demo pages did this: Menu Separator
Here’s how I solved it.
The problem can be addressed with the Web Developer add on.
Normally its Disable / Disable Minimum Font Size is checked. If it gets unchecked, Ext behaves very badly.
Check it and then reload the page; menu separators will be back to normal.