There is probably just a about:config setting. I can't find any settings in Organize Bookmarks. Thank You!
EDIT Sorry, I misread the question - this answer is irrelevant...
Hi
I have never heard of any tweak inceasing the size of toolbars. However you can increase the text size within the toolbar, which accomplishes something similar...I think
Here is a before and after screenshot, one with the default text size and the other with the text size increased 200%. Below them is the code you put in your userchrome.css file. (ask if you need help with this)
You can change the percentage increase/decrease accordingly:
Before:
After:
and the code:
/* Shrink/increase tab titles by set % */.tabbrowser-tabs .tab-text { font-size: 200%;}
Not exactly what your looking for I know, but it might be of help.
The answer!