Ohodnoceno 1 z 5 hvězdiček

It doesn't do anything even with Options/Content/Advanced set to override page test font and size.

You need to choose a page with very small font to see an effect.
Don't forget to press Ctrl+Shift+0 after installing add-on.

Doesn't work Ohodnoceno 5 z 5 hvězdiček

Great, just what I was looking for. Unfortunately CTRL-SHIFT-O doesn't work. Using Firefox 28 beta.

Not letter O, it's number zero key.