Comment 44 for bug 41411

@Forest note on user level font.conf works around the Nimbus rendering issues.

just run fc-cache -f -v to update the fontconfig cache afterwards and refresh your browser page.

Oddly this problem only appears in Firefox 3&4. Both Chrome and Opera correctly detect that Helvetica is not installed and default to the backup fonts (usually Arial, Verdana, Tahoma etc in the case of CSS - " font-family : Helvetica, Arial, Tahoma, Verdana, sans-serif; ")

The trouble with this default aliasing to Nimbus is that for CSS authors who specify Helvetica as a first level font choice on their sites, any users with the default setup viewing the site will get Nimbus font rendering. The replacement of Helvetica with Nimbus fools Firefox into thinking that Helvetica is installed on the system.
Comparing Helvetica on a Mac with 'Helvetica (Nimbus)' in FF/kUbuntu is no contest and there is no way that Nimbus is a valid replacement for Helvetica.

Taking out the Helvetica rule in CSS using Firebug in FF3 and 4 will 'fix' the font as it will fallback to backup fonts like Arial or Verdana which are part of the MS font pack.