Tried to make your own user defined stylesheet yet? If you're familiar with html and css, you could try making a user defined stylesheet which you can use to override the style settings for a website. So in your case, you could make a stylesheet that makes the font bigger in those selection boxes and buttons only. I'm not on my machine right now, but in your mozilla firefox profile directory, there should be a folder chrome/ where you can make a userContent.css.
http://www.mozilla.org/unix/customizing.html#usercss
search for more on google.
I would imagine, it'd be something like this, haven't tested:
Code:
input, option {
font-size: 1.25em
}
I'm sure you can come up with something more specific that wouldn't mess up other webpages.