The latest version of Firefox has SVG built in and enabled by default. Go to http://www.mozilla.org/projects/firefox/ and download Firefox 1.5 (or later). This should work on Windows, Mac OS X and Linux.
On Debian Linux, the standard version of Mozilla is SVG-enabled, but (as noted below) you may still need to install the Adobe SVG viewer as well.
Note: In order for ASCIIsvg to work in older versions of SVG-enabled Mozilla browsers, one still needs to install the Adobe SVG viewer 3.01 and copy the two files NPSVG3.dll, NPSVG3.zip from c:\Program Files\Common Files\Adobe\SVG Viewer 3.0\Plugins to the Mozilla or Firefox plugin folder (usually c:\Program Files\mozilla.org\Mozilla\bin\plugins or c:\Program Files\Mozilla Firefox\plugins).