This question is an exact duplicate of:
See the following screen-shot:
That's Firefox 5 on Ubuntu 11.04. It could be that I don't have the intended font and may be falling back to one that has different character widths?
Specified font family:
Trebuchet MS,Liberation Sans,DejaVu Sans,sans-serif. It appears to be attempting to use Liberation Sans. When I remove that using firebug, it gets even worse (presumably because it uses the even wider OS default).
Ubuntu doesn't appear to be doing any sneaky swapping of fonts:
>fc-match "Liberation Sans" LiberationSans-Regular.ttf: "Liberation Sans" "Regular"
Not a huge problem, but I thought I ought to point it out.