Are you sure that this is a different font than the system default one? It seems that you don't have Arial on your system, so the browsers are falling back to the system default font. Remember, Arial isn't a free font, so it isn't bundled with GPL operation systems like Ubuntu. And Helvetica is a popular font in professional design, but it doesn't come bundled with Windows or with popular software packages, so most systems don't have it at all.
If you want to use another, nicer font as the system font, open "Appearance" under "Preferences", and on one of the tabs, you can change the default font for the whole system. If you want to keep your system font, but want to the pages written for Arial rendered in Arial, you have to find an Arial font file (the easiest place is the font directory of a Windows installation, although this choice probably doesn't agree with copyright law) and copy it into ~/.fonts on your computer. Probably you'll also need to restart the computer or at least the X server.