This issue can be caused by an old bitmap version of the Helvetica or Geneva font or (bitmap) fonts that Firefox can't display in that size.

Firefox can't display some old bitmap fonts in a larger size and displays gibberish instead.

You can test that by zooming out (View > Zoom > Zoom Out, Ctrl -) to make the text smaller.

Uninstall (remove) all variants of that not working font to make Firefox use another font or see if you can find a True type version that doesn't show the problem.

Zooming doesn't solve the problem and I don't know how to uninstall fonts.

You can check and uninstall fonts via Control Panel > Fonts or search the fonts folder in C:\Windows\Fonts

http://windows.microsoft.com/en-US/windows-vista/Install-or-uninstall-fonts

I have the same problem which is one reason I still use IE8 (which has its own problems). I change my font to be bigger, and I use my toolbar on the side. Firefox apparently doesn't like this and squooshes the page together.

I could do all sorts of research to figure out what is wrong, reconfigure Firefox, use the fonts it wants, install, zoom, uninstall fonts, and perhaps find apps to change the way it looks. I probably could set up my computer to satisfy the needs of Firefox.

Nah. I like my computer to work for me! So, I have to go to IE a lot. I use FF for some things. IE for others.

Good luckl