I don't like Ubuntu Firefox default font, I want to put Arial font like in Windows. What should I do for that(I don't have Arial font in Firefox fonts list)?
Arial is in the msttcorefonts (Microsoft TrueType core fonts) package, which you can install by typing this line into the console:
If you want to change the font in Firefox only, open up Firefox, go to Edit > Preferences, select the Content tab and choose Arial as the default font. If you want Ubuntu to use it elsewhere, open up System > Preferences > Appearance in your top panel and select the Fonts tab. You can now pick Arial for any group you want.