I came out to curious situation with menu on http://jdem.cz/vzcy9 . I was developing it under Windows 7, FF 13/14, IE 9, Opera 11 and it displays fine there. But on any Linux browser the menu there is spread to two lines (see the image below).
I’ve tried to set smaller margin and padding of every menu link but it didn’t help.
Do someone have some idea about where could be the problem?

Thank you for your advices. It really was a problem with font (Georgia) which is not present on Linux. It’s Linux equivalent was much more “wider” which broke the layout.
I added:
Which solved the problem.