How to set "Text size =" automatically?

[Steve Snyder]

I'm using Mozilla v0.9.3 on a Linux box.  I find that setting the text size 
to 120% (View>>Text Size>>120%) is much more readable to my aging eyes than 
the default 100%.  I've tried playing with the font sizes 
(Edit>>Preferences>Appearance>>Fonts) but haven't been able to get the same 
favorable appearance.

It's annoying to have to manually set the text size on each browser windows 
that I open.  Is there a way to tell Mozilla to always use TextSize=120%?

Thanks.